Problem to crop edges when creating pdf using standalone package

Question

I am trying to make a pdf figure comprised of several other figures arranged in a table manner. I am using the standalone documentclass. However the results I get are far from optimal. I am using Texstudio for Windows, PDFLatex command on Miktek 2.9: pdflatex.exe -synctex=1 -interaction=nonstopmode %.tex

When using the first two options for the standalone package (in the MWE) I obtain a pdf where the upper and lower sides are cropped but not on left and right sides. When using the varwidth option to true, I obtain the desired cropped edges but the caption titles ((a),(b),...) are all left justified and brackets are gone. Whenever I remove the preview=true option I am filled with errors.

I attach the MWE; in the original, figure (d) is twice as tall as this black box.

%\documentclass[border=2pt,crop=true,preview=true]{standalone} % works but spaces on sides
%\documentclass[border=2pt,preview=true]{standalone} % works but spaces on sides
%\documentclass[varwidth=true,preview=true]{standalone} % works reduces side spaces, but captions ragged left without brackets
\documentclass[varwidth=true]{standalone} % same

\usepackage[demo]{graphicx}
\usepackage{tabularx}
\usepackage{subcaption}

\renewcommand{\tabularxcolumn}[1]{m{#1}} 
\begin{document}
\begin{figure}
    \centering
    \begin{tabularx}{0.75\textwidth}{X X} % @{}
        \begin{tabular}{c} 
            \begin{subfigure}[]{0.99\linewidth}
            \centering
            \includegraphics[width=1\textwidth]{}
            \caption{}
            \label{a}
            \end{subfigure} \\ 
            \begin{subfigure}[]{0.99\linewidth}
            \centering
            \includegraphics[width=1\textwidth]{}
            \caption{}
            \label{b}
            \end{subfigure} \\ 
            \begin{subfigure}[]{0.99\linewidth}
            \centering
            \includegraphics[width=1\textwidth]{}
            \caption{}
            \label{c}
            \end{subfigure}
        \end{tabular}
        & 
        \begin{tabular}{c} 
            \begin{subfigure}[]{0.99\linewidth}
            \centering
            \includegraphics[width=0.99\textwidth]{}
            \caption{}
            \label{d}
            \end{subfigure}
        \end{tabular}
    \end{tabularx}
\end{figure}
\end{document}

I would appreciate your comments on what is going on here and what can be done to solve this problem.

Answer 1

First, I have used @{} at appropriate places. Then the subfigure and figure environments are removed. Then it is time to load caption package and use its \captionof macro. This macro is enclosed within a \parbox (a need).

Additionally the subcaption is loaded like

\usepackage[labelformat=simple]{subcaption}
\renewcommand\thesubfigure{(\alph{subfigure})}

So the full code will be

\documentclass{standalone}

\usepackage[demo]{graphicx}
\usepackage{tabularx}
\usepackage{caption}
\usepackage[labelformat=simple]{subcaption}
\renewcommand\thesubfigure{(\alph{subfigure})}

\renewcommand{\tabularxcolumn}[1]{m{#1}}
\begin{document}
    \begin{tabularx}{0.75\textwidth}{@{}X X@{}} % @{}
        \begin{tabular}{@{}c}
            \parbox{\linewidth}{%
            \includegraphics[width=\linewidth]{}
            \captionof{subfigure}{}
            \label{a}
            }
            \\
            \parbox{\linewidth}{%
            \includegraphics[width=\linewidth]{}
            \captionof{subfigure}{}
            \label{b}
            }
            \\
            \parbox{\linewidth}{%
            \includegraphics[width=\linewidth]{}
            \captionof{subfigure}{}
            \label{c}
            }
        \end{tabular}
        &
        \begin{tabular}{@{}c}
            \parbox{\linewidth}{%
            \includegraphics[width=\linewidth]{}
            \captionof{subfigure}{}
            \label{d}
            }
        \end{tabular}
    \end{tabularx}
\end{document}