# Captioning some tikz figures for a multiple choice question

I'd like to create a question with for possible choices. Each choice will be a tikz figure. How can I insert the caption to simply look like

(A) (B)
(C) (D)


Directly under each of the figures?

La d\'{e}riv\'{e}e d'une fonction est $f'(x) = (x+1)(x-3)$.
Lequel des \'{e}l\'{e}ments suivants pourrait \^{e}tre un croquis de la fonction?

\begin{tikzpicture}
\begin{axis}[
ticks=none, axis x line=middle, axis y line=middle,
ymin=-20, ymax=15, ylabel=$y$,
xmin=-4, xmax=4, xlabel=$x$,
]

\draw[dashed] (-1,15) -- (-1,-14) node[below]{$x=-1$}  ;
\draw[dashed] (3,15) -- (3,-14) node[below]{$x=3$};
\end{axis}
\end{tikzpicture}
\begin{tikzpicture}
\begin{axis}[
ticks=none, axis x line=middle, axis y line=middle,
ymin=-20, ymax=15, ylabel=$y$,
xmin=-4, xmax=4, xlabel=$x$,
]

\draw[dashed] (-1,15) -- (-1,-14) node[below]{$x=-1$}  ;
\draw[dashed] (3,15) -- (3,-14) node[below]{$x=3$};
\end{axis}
\end{tikzpicture}

\begin{tikzpicture}
\begin{axis}[
ticks=none, axis x line=middle, axis y line=middle,
ymin=-20, ymax=15, ylabel=$y$,
xmin=-4, xmax=4, xlabel=$x$,
]

\draw[dashed] (-1,15) -- (-1,-14) node[below]{$x=-1$}  ;
\draw[dashed] (3,15) -- (3,-14) node[below]{$x=3$};
\end{axis}
\end{tikzpicture}
\begin{tikzpicture}
\begin{axis}[
ticks=none, axis x line=middle, axis y line=middle,
ymin=-20, ymax=15, ylabel=$y$,
xmin=-4, xmax=4, xlabel=$x$,
]

\draw[dashed] (-1,15) -- (-1,-14) node[below]{$x=-1$}  ;
\draw[dashed] (3,15) -- (3,-14) node[below]{$x=3$};
\end{axis}
\end{tikzpicture}


You could use a package like subcaption to create subfigures, but you could also just add a node at the bottom of the tikzpicture as in this example. If you want a bit more space between axis and label, modify the newcommand to say e.g. below=3mm instead of just below.

\documentclass{article}
\usepackage{pgfplots}
\pgfplotsset{compat=1.14,width=6cm}
\newcommand\tikzsublabel[1]{\node [below] at (current bounding box.south) {(#1)};}
\begin{document}

La d\'{e}riv\'{e}e d'une fonction est $f'(x) = (x+1)(x-3)$.
Lequel des \'{e}l\'{e}ments suivants pourrait \^{e}tre un croquis de la fonction?
\begin{center}
\begin{tikzpicture}
\begin{axis}[
ticks=none, axis x line=middle, axis y line=middle,
ymin=-20, ymax=15, ylabel=$y$,
xmin=-4, xmax=4, xlabel=$x$,
]

\draw[dashed] (-1,15) -- (-1,-14) node[below]{$x=-1$}  ;
\draw[dashed] (3,15) -- (3,-14) node[below]{$x=3$};
\end{axis}
\tikzsublabel{A}
\end{tikzpicture}
\begin{tikzpicture}
\begin{axis}[
ticks=none, axis x line=middle, axis y line=middle,
ymin=-20, ymax=15, ylabel=$y$,
xmin=-4, xmax=4, xlabel=$x$,
]

\draw[dashed] (-1,15) -- (-1,-14) node[below]{$x=-1$}  ;
\draw[dashed] (3,15) -- (3,-14) node[below]{$x=3$};
\end{axis}
\tikzsublabel{B}
\end{tikzpicture}

\begin{tikzpicture}
\begin{axis}[
ticks=none, axis x line=middle, axis y line=middle,
ymin=-20, ymax=15, ylabel=$y$,
xmin=-4, xmax=4, xlabel=$x$,
]

\draw[dashed] (-1,15) -- (-1,-14) node[below]{$x=-1$}  ;
\draw[dashed] (3,15) -- (3,-14) node[below]{$x=3$};
\end{axis}
\tikzsublabel{C}
\end{tikzpicture}
\begin{tikzpicture}
\begin{axis}[
ticks=none, axis x line=middle, axis y line=middle,
ymin=-20, ymax=15, ylabel=$y$,
xmin=-4, xmax=4, xlabel=$x$,
]

\draw[dashed] (-1,15) -- (-1,-14) node[below]{$x=-1$}  ;
\draw[dashed] (3,15) -- (3,-14) node[below]{$x=3$};
\end{axis}
\tikzsublabel{D}
\end{tikzpicture}
\end{center}
\end{document}