8.tex

\documentclass{article}

\usepackage{graphicx}

\usepackage{subcaption}

\begin{document}

\begin{figure}[h]

\centering

\begin{subfigure}[b]{0.45\textwidth}

\centering

\fbox{\rule{0pt}{5cm}\rule{5cm}{0pt}} % placeholder (replace with \includegraphics)

\caption{Subfigure 1 caption}

\label{fig:sub1}

\end{subfigure}

\hfill

\begin{subfigure}[b]{0.45\textwidth}

\centering

\fbox{\rule{0pt}{5cm}\rule{5cm}{0pt}}

\caption{Subfigure 2 caption}

\label{fig:sub2}

\end{subfigure}

\vspace{1em}

\begin{subfigure}[b]{0.45\textwidth}

\centering

\fbox{\rule{0pt}{5cm}\rule{5cm}{0pt}}

\caption{Subfigure 3 caption}

\label{fig:sub3}

\end{subfigure}

\hfill

\begin{subfigure}[b]{0.45\textwidth}

\centering

\fbox{\rule{0pt}{5cm}\rule{5cm}{0pt}}

\caption{Subfigure 4 caption}

\label{fig:sub4}

\end{subfigure}

\caption{This is a figure containing several subfigures.}

\label{fig:subfigs}

\end{figure}

In the text, you can refer to subfigures of Figure~\ref{fig:subfigs}

as \ref{fig:sub1}, \ref{fig:sub2}, \ref{fig:sub3}, and \ref{fig:sub4},

and to the sub-index as (a), (b), (c) and (d).

\end{document}