To make your code compilable, a \\ right before the last \noalign{\smallskip}\hline is missing. Having added this missing piece of code, let's focus on improving the output. With the help of the tabularx package you can easily make sure a table fits into the textwidth and horizontal lines from the booktabs package could be used instead of \noalign{\smallskip}\hline. Applying thouse changes, one ends up with:

Depending on your documentclass, the margin and font sizes as well as the individual contents of the table, you may want to alter the width of the X type columns. Table layout with tabularx (column widths: 50%|25%|25%) contains useful information on that. Text in narrower columns often looks better left aligned instead of justified and if you want left-aligned text in your X type columns, you could use >{\raggedright\arraybackslash}X instead of X.
\documentclass{article}
\usepackage{booktabs}
\usepackage{tabularx}
\begin{document}
\begin{table}
\begin{tabularx}{\linewidth}{lXlX}
\toprule
Study & CNN & Task & Optimization \
\midrule
authors & CNN and Extreme Learning Machine & Image Classification & Ensemble methods with CNN.\
\bottomrule
\end{tabularx}
\end{table}
\end{document}
makecellpackage and its eponymous command provides the possibility to use\\in standard columns. – Bernard Jun 29 '21 at 23:13