This looks like an ideal task for tasks (here in combination with exsheets which as of May 2017 is superseded by xsim):

\documentclass{article}
\usepackage{exsheets}
% this sets up the question headings to be a simple run-in number:
\SetupExSheets{
headings = runin-nr ,
headings-format = \normalfont
}
\usepackage{tasks}
% this creates a new environment `choices' where the single
% choices are indicated with \choice and are labelled (a), (b), ...:
\NewTasks[counter-format=(tsk[a]),label-width=2em]{choices}[\choice]
\begin{document}
\begin{question}
Which of the following numbers is the largest?\\
$2^{3^4}$, $2^{4^3}$, $3^{2^4}$, $3^{4^2}$, $4^{2^3}$, $4^{3^2}$.
\begin{choices}
\choice $2^{3^4}$
\choice $3^{4^2}$
\choice $4^{3^2}$
\choice $4^{2^3}$
\end{choices}
\end{question}
\begin{question}
Which of the following numbers is the largest?\\
$2^{3^4}$, $2^{4^3}$, $3^{2^4}$, $3^{4^2}$, $4^{2^3}$, $4^{3^2}$.
% the optional argument in parentheses determines the number of columns:
\begin{choices}(2)
\choice $2^{3^4}$
\choice $3^{4^2}$
\choice $4^{3^2}$
\choice $4^{2^3}$
\end{choices}
\end{question}
\begin{question}
Which of the following numbers is the largest?\\
$2^{3^4}$, $2^{4^3}$, $3^{2^4}$, $3^{4^2}$, $4^{2^3}$, $4^{3^2}$.
\begin{choices}(4)
\choice $2^{3^4}$
\choice $3^{4^2}$
\choice $4^{3^2}$
\choice $4^{2^3}$
\end{choices}
\end{question}
\end{document}
How do you accept an answer?. – Gonzalo Medina Apr 05 '13 at 03:29