Here is a modification of Claudio's answer. With this modification, if you don't use the answers option of the exam class the correct answer is not boxed, you can give the exam to your students, the result is:

If the answers option is used (the syntax is
\documentclass[answers]{exam}
) then the correct answers are boxed. The result is:

The code is
\documentclass[12pt]{exam} %Use the answers option to box correct answers
\usepackage{tikz}
\usetikzlibrary{calc,shapes}
\newcommand{\tikzmark}[1]{\tikz[overlay,remember picture] \node (#1) {};}
\newcommand{\DrawSmartBox}[1][red]{%
\tikz[overlay,remember picture]{
\draw[#1]
($(bl)+(-1.75em,1em)$) rectangle
($(br)+(0.2em,-0.4em)$);}
}
%Here is the modification
\ifprintanswers
% \MyCorrectSmartChoice:
% #1 optional argument: aspect customization
% #2 mandatory argument: the answer
\newcommand{\MyCorrectSmartChoice}[2][black]{\CorrectChoice \tikzmark{bl}#2\tikzmark{br}\DrawSmartBox[#1]}
\else
\newcommand{\MyCorrectSmartChoice}[2][black]{\CorrectChoice #1}
\fi
\begin{document}
\begin{questions}
\question Who is an idiot?
\begin{choices}
\choice Not the writer of this post
\MyCorrectSmartChoice[thick,rounded corners,red]{The writer of this post}
\choice Who read this post
\end{choices}
\end{questions}
\end{document}