How to make use hline for only the first two column?

Question

I want to use horizontal line for the first two column. See my picture for details.

\documentclass[12pt,letterpaper]{article}    
\usepackage[left=20mm,top=30mm,bottom=30mm,right=20mm]{geometry}
\begin{document}
\begin{table}
\centering
\resizebox{0.7\columnwidth }{!}{%
\begin{tabular}{ |p{5cm}|p{5cm}|p{6cm}|  }
\hline
\hline
\textbf{Portion of Scenario} & \textbf{Description} & \textbf{Possible Values} \
\hline
Sources & This specify where the fault comes from & Internal/external: people, hardware, software, physical infrastructure, physical environment\
\hline
Stimulus & The stimulus to an availability scenario is a fault.   & Fault: omission, crash, incorrect timing, incorrect response \
\hline
Artifact & This specifies which portions of the system are responsible for and affected by the fault.& Processors, communication channels, storage, processes, affected artifacts in the system's
environment \
\hline
Environment    &We may be interested in not only how a system behaves in its "normal" environment, but also how it behaves in situations such as when it is already recovering from a fault. & Normal operation, startup, shutdown, repair mode, degraded operation, overloaded operation \
\hline
Response & The most commonly desired response is to prevent the fault from becoming a failure, but other responses may also be important, such as notifying people or logging the fault for later analysis. This section specifies the desired system response. & Prevent the fault from becoming a failure
\newline
Detect the fault:
\begin{itemize}
\item Log the fault
\item Notify the appropriate entities (people or systems)
\item Recover from the fault
\item Disable the source of events causing the fault
\item Be temporarily unavailable while a repair is
being effected
\item Fix or mask the fault/failure or contain the damage it causes
\item Operate in a degraded mode while a repair is
being effected
\item Time or time interval when the system must be available
\item Availability percentage (e.g., 99.999 percent)
\item Time to detect the fault
\item Time to repair the fault
\item Time or time interval in which system can be in degraded mode
\item Proportion (e.g., 99 percent) or rate (e.g., up to 100 per second) of a certain class of faults that the system prevents, or handles without failing 
\end{itemize}
\
Response measure & We may focus on a number of measures of availability, depending on the criticality of the service being provided.  \
\hline
\end{tabular}
}
\end{table}
\end{document}

Answer 1

I think the main issue with your table is that the font is very small due to the \resizebox instruction. It looks like you've inserted the \resizebox instruction in order to make the table fit on a single page.

I would like to recommend a different approach to making the table fit on the page: Use a tabularx environment, let the three columns have different widths, and use the machinery of the enumitem package to make the itemize environment in the third column take up a lot less space.

\documentclass[12pt,letterpaper]{article}    
\usepackage[hmargin=20mm, vmargin=30mm]{geometry}
% new code:
\usepackage{enumitem}
\usepackage{tabularx,ragged2e}
% See section 4.3, "column widths", of user guide of 'tabularx' package:
\newcolumntype{L}[1]{>{\RaggedRight\hsize=#1\hsize\linewidth=\hsize}X}
\begin{document}
\begin{table}[p!]
\small % for a 10% linear font size reduction
\begin{tabularx}{\textwidth}{ |L{0.45}|L{0.9}|L{1.65}| }
\hline
\textbf{Portion of Scenario} & 
\textbf{Description} & 
\textbf{Possible Values} 
\ \hline
Sources & 
This specifies where the fault comes from. & 
Internal/external: people, hardware, software, physical infrastructure, physical environment
\ \hline
Stimulus & 
The stimulus to an availability scenario is a fault. & 
Fault: omission, crash, incorrect timing, incorrect response 
\ \hline
Artifact & 
This specifies which portions of the system are responsible for and affected by the fault. & 
Processors, communication channels, storage, processes, affected artifacts in the system's environment 
\ \hline
Environment &
We may be interested in not only how a system behaves in its ``normal'' environment, but also how it behaves in situations such as when it is already recovering from a fault. & 
Normal operation, startup, shutdown, repair mode, degraded operation, overloaded operation 
\ \hline
Response & 
The most commonly desired response is to prevent the fault from becoming a failure, but other responses may also be important, such as notifying people or logging the fault for later analysis. This section specifies the desired system response. & 
Prevent the fault from becoming a failure. \newline
Detect the fault: 
\begin{itemize}[nosep, left=0pt]
\item Log the fault
\item Notify the appropriate entities (people or systems)
\item Recover from the fault
\item Disable the source of events causing the fault
\item Be temporarily unavailable while a repair is being effected
\item Fix or mask the fault/failure or contain the damage it causes
\item Operate in a degraded mode while a repair is being effected
\item Time or time interval when the system must be available
\item Availability percentage (e.g., 99.999 percent)
\item Time to detect the fault
\item Time to repair the fault
\item Time or time interval in which system can be in degraded mode
\item Proportion (e.g., 99 percent) or rate (e.g., up to 100 per second) of a certain class of faults that the system prevents, or handles without failing.
\end{itemize}
\ \cline{1-2}
Response measure & 
We may focus on a number of measures of availability, depending on the criticality of the service being provided. & 
\ \hline
\end{tabularx}
\end{table}
\end{document}

Answer 2

With tabularray and the library varwidth section 5.8 in the documentation, i replaced \resizebox with sriptsize \resizebox doesn't seem like a good idea. see this post tex.stackexchange.com/questions/425453/… and Using adjustbox withouth changing fontsize

EDIT: with Mico's recommendations, and the idea of ​​setting up the itemize environment with the enumitem package

The code

\documentclass[12pt,letterpaper]{article}
%\usepackage[left=20mm,top=30mm,bottom=30mm,right=20mm]{geometry}
\usepackage[hmargin=20mm, vmargin=30mm]{geometry}%<-- Mico's comment
\usepackage{tabularray,varwidth,enumitem}%<-- package enimitem
\pagestyle{empty}
\begin{document}
%\begin{scriptsize}%<-- Mico's explanation
{\scriptsize
\begin{tblr}{
    colspec = {*{2}{Q[wd=5cm]} Q[wd=6cm]},
    row{1} = {font=\bfseries},
    vlines,
}
\hline
Portion of Scenario & Description & Possible Values \\
\hline
Sources & This specify where the fault comes from & Internal/external: people, hardware, software, physical infrastructure, physical environment\\
\hline
Stimulus & The stimulus to an availability scenario is a fault.   & Fault: omission, crash, incorrect timing, incorrect response \\
\hline
Artifact & This specifies which portions of the system are responsible for and affected by the fault.& Processors, communication channels, storage, processes, affected artifacts in the system's
environment \\
\hline
Environment    &We may be interested in not only how a system behaves in its "normal" environment, but also how it behaves in situations such as when it is already recovering from a fault. & Normal operation, startup, shutdown, repair mode, degraded operation, overloaded operation \\
\hline
Response & The most commonly desired response is to prevent the fault from becoming a failure, but other responses may also be important, such as notifying people or logging the fault for later analysis. This section specifies the desired system response. & Prevent the fault from becoming a failure
\newline
Detect the fault:
\begin{itemize}[nosep, left=0pt]
\item Log the fault
\item Notify the appropriate entities (people or systems)
\item Recover from the fault
\item Disable the source of events causing the fault
\item Be temporarily unavailable while a repair is
being effected
\item Fix or mask the fault/failure or contain the damage it causes
\item Operate in a degraded mode while a repair is
being effected
\item Time or time interval when the system must be available
\item Availability percentage (e.g., 99.999 percent)
\item Time to detect the fault
\item Time to repair the fault
\item Time or time interval in which system can be in degraded mode
\item Proportion (e.g., 99 percent) or rate (e.g., up to 100 per second) of a certain class of faults that the system prevents, or handles without failing 
\end{itemize}
\\
\cline{1-2}
Response measure & We may focus on a number of measures of availability, depending on the criticality of the service being provided.  \\
\hline
\end{tblr}
}%<-- end of scriptsize
%\end{scriptsize}%
\end{document}