Controlling subfigure captions and subfigure placement

The optional argument for the subfigure environment provides the placement of the sub-caption anchor. This is used to align the sub-captions vertically. Here is a visual:

\documentclass{article}

\usepackage{graphicx,subcaption}

\begin{document}

\begin{figure}
  \centering
  \texttt{subfigure} with \texttt{[t]}op alignment

  \medskip

  \begin{subfigure}[t]{.3\linewidth}
    \centering\includegraphics[width=.5\linewidth]{example-image-a}
    \caption{This is a sub-caption. This is a sub-caption. This is a sub-caption}
  \end{subfigure}
  \begin{subfigure}[t]{.3\linewidth}
    \centering\includegraphics[width=.5\linewidth]{example-image-b}
    \caption{This is a sub-caption.}
  \end{subfigure}
  \begin{subfigure}[t]{.3\linewidth}
    \centering\includegraphics[width=.5\linewidth]{example-image-c}
    \caption{This is a sub-caption. This is a sub-caption.}
  \end{subfigure}

  \bigskip\hrulefill\bigskip

  \texttt{subfigure} with \texttt{[c]}entre alignment (default)

  \medskip

  \begin{subfigure}{.3\linewidth}
    \centering\includegraphics[width=.5\linewidth]{example-image-a}
    \caption{This is a sub-caption. This is a sub-caption. This is a sub-caption}
  \end{subfigure}
  \begin{subfigure}{.3\linewidth}
    \centering\includegraphics[width=.5\linewidth]{example-image-b}
    \caption{This is a sub-caption.}
  \end{subfigure}
  \begin{subfigure}{.3\linewidth}
    \centering\includegraphics[width=.5\linewidth]{example-image-c}
    \caption{This is a sub-caption. This is a sub-caption.}
  \end{subfigure}

  \bigskip\hrulefill\bigskip

  \texttt{subfigure} with \texttt{[b]}ottom alignment

  \medskip

  \begin{subfigure}[b]{.3\linewidth}
    \centering\includegraphics[width=.5\linewidth]{example-image-a}
    \caption{This is a sub-caption. This is a sub-caption. This is a sub-caption}
  \end{subfigure}
  \begin{subfigure}[b]{.3\linewidth}
    \centering\includegraphics[width=.5\linewidth]{example-image-b}
    \caption{This is a sub-caption.}
  \end{subfigure}
  \begin{subfigure}[b]{.3\linewidth}
    \centering\includegraphics[width=.5\linewidth]{example-image-c}
    \caption{This is a sub-caption. This is a sub-caption.}
  \end{subfigure}
\end{figure}

\end{document}

So I'd use a [t]op alignment. Additionally, you can spread out the figures using a number of options.

\documentclass{article}

\usepackage{graphicx,subcaption}
\usepackage[margin=1cm,showframe]{geometry}% Just for this example

\begin{document}

\begin{figure}
  \centering

  Fixed separation using \verb|\quad| (or \verb|\qquad|, or \verb|\hspace|)

  \medskip

  \begin{subfigure}[t]{.4\linewidth}
    \centering\includegraphics[width=.5\linewidth]{example-image-a}
    \caption{This is a sub-caption. This is a sub-caption. This is a sub-caption}
  \end{subfigure}\quad
  \begin{subfigure}[t]{.4\linewidth}
    \centering\includegraphics[width=.5\linewidth]{example-image-b}
    \caption{This is a sub-caption.}
  \end{subfigure}

  \bigskip\hrulefill\bigskip

  Even spread via \verb|\hfill|

  \medskip

  \mbox{}\hfill  
  \begin{subfigure}[t]{.4\linewidth}
    \centering\includegraphics[width=.5\linewidth]{example-image-a}
    \caption{This is a sub-caption. This is a sub-caption. This is a sub-caption}
  \end{subfigure}\hfill
  \begin{subfigure}[t]{.4\linewidth}
    \centering\includegraphics[width=.5\linewidth]{example-image-b}
    \caption{This is a sub-caption.}
  \end{subfigure}
  \hfill\mbox{}

  \bigskip\hrulefill\bigskip

  Full spread via \verb|\hfill|

  \medskip

  \begin{subfigure}[t]{.4\linewidth}
    \centering\includegraphics[width=.5\linewidth]{example-image-a}
    \caption{This is a sub-caption. This is a sub-caption. This is a sub-caption}
  \end{subfigure}\hfill
  \begin{subfigure}[t]{.4\linewidth}
    \centering\includegraphics[width=.5\linewidth]{example-image-b}
    \caption{This is a sub-caption.}
  \end{subfigure}

\end{figure}

\end{document}

If your images have different vertical heights and you want to align those as well, you can raise content into place; the default alignment of images would be at the baseline:

\documentclass{article}

\usepackage{graphicx,subcaption}
\newsavebox{\imagebox}

\begin{document}

\begin{figure}
  \centering

  Default

  \medskip

  \begin{subfigure}[t]{.4\linewidth}
    \centering\includegraphics[width=.5\linewidth,height=75pt]{example-image-a}
    \caption{This is a sub-caption. This is a sub-caption. This is a sub-caption}
  \end{subfigure}\qquad
  \begin{subfigure}[t]{.4\linewidth}
    \centering\includegraphics[width=.5\linewidth,height=5pc]{example-image-b}
    \caption{This is a sub-caption.}
  \end{subfigure}

  \bigskip\hrulefill\bigskip

  \verb|\raisebox| to vertical top

  \medskip

  % Store largest image in a box
  \savebox{\imagebox}{\includegraphics[width=.2\linewidth,height=75pt]{example-image-a}}%
  \begin{subfigure}[t]{.4\linewidth}
    \centering\usebox{\imagebox}% Place largest image
    \caption{This is a sub-caption. This is a sub-caption. This is a sub-caption}
  \end{subfigure}\qquad
  \begin{subfigure}[t]{.4\linewidth}
    \centering\raisebox{\dimexpr\ht\imagebox-\height}{% Raise smaller image into place
      \includegraphics[width=.5\linewidth,height=5pc]{example-image-b}}%
    \caption{This is a sub-caption.}
  \end{subfigure}

  \bigskip\hrulefill\bigskip

  \verb|\raisebox| to vertical centre

  \medskip

  % Store largest image in a box
  \savebox{\imagebox}{\includegraphics[width=.2\linewidth,height=75pt]{example-image-a}}%
  \begin{subfigure}[t]{.4\linewidth}
    \centering\usebox{\imagebox}% Place largest image
    \caption{This is a sub-caption. This is a sub-caption. This is a sub-caption}
  \end{subfigure}\qquad
  \begin{subfigure}[t]{.4\linewidth}
    \centering\raisebox{\dimexpr.5\ht\imagebox-.5\height}{% Raise smaller image into place
      \includegraphics[width=.5\linewidth,height=5pc]{example-image-b}}%
    \caption{This is a sub-caption.}
  \end{subfigure}

\end{figure}

\end{document}

• Use the subfigure option [t] instead of [b].

• To add some horizontal space between the figures, put \quad, \qquad, \hfill or some \hspace command between the subfigure environments.

.

\documentclass{article}
\usepackage{graphicx, caption, subcaption}
\begin{document}
  This article is about geometrical figures.
  \begin{figure}
    \begin{subfigure}[t]{0.4\textwidth}
      \includegraphics[width=\textwidth]{example-image-a}
      \caption{The rectangle is a complicated geometrical figure that has 4 edges and 4 vertices while a star is an even more complex geometrical figure}
    \end{subfigure}
    \hfill
    \begin{subfigure}[t]{0.35\textwidth}
      \includegraphics[width=\textwidth]{example-image-b}
      \caption{A simple circle and a spiral}
    \end{subfigure}
    \caption{Geometrical figures}
  \end{figure}
\end{document}