All that I did was to add &\\[-\normalbaselineskip]& between the 2nd and last columns on every row.
\documentclass[]{article}
\usepackage{multirow}
\usepackage{multicol}
\usepackage{array}
\begin{document}
\pagenumbering{gobble}
\begin{center}
\begin{tabular}{cp{7.5cm}p{2.4cm}}
\textbf{L4} &
For a sound explanation of both budget lines and indifference curves and a clear link to the separate demand curve, sound explanation of link between utility, price and demand, with accurate clear diagrams and a clear understanding of the principles involved.
&\\[-\normalbaselineskip]&
&\textbf{[9--12]}\\
\textbf{L4} &
For a sound explanation of both budget lines and indifference curves and a clear link to the separate demand curve, sound explanation of link between utility, price and demand, with accurate clear diagrams and a clear understanding of the principles involved.
&\\[-\normalbaselineskip]&
&\textbf{[9--12]}\\
\end{tabular}
\end{center}
\end{document}

This can be codified in macro form (though it is particular to a 3-column tabular. Here, \lastbit{} is the macro:
\documentclass[]{article}
\usepackage{multirow}
\usepackage{multicol}
\usepackage{array}
\def\lastbit#1{\\[-\arraystretch\normalbaselineskip]&&\textbf{#1}}
\begin{document}
\pagenumbering{gobble}
\begin{center}
\begin{tabular}{cp{7.5cm}p{2.4cm}}
\textbf{L4} &
For a sound explanation of both budget lines and indifference curves and a clear link to the separate demand curve, sound explanation of link between utility, price and demand, with accurate clear diagrams and a clear understanding of the principles involved.
&\lastbit{[9--12]}\\
\textbf{L4} &
For a sound explanation of both budget lines and indifference curves and a clear link to the separate demand curve, sound explanation of link between utility, price and demand, with accurate clear diagrams and a clear understanding of the principles involved.
&\lastbit{[9--12]}\\
\end{tabular}
\end{center}
\end{document}