Exam Class: Make math expressions in \correctchoice bold

The best solution would be to use \correctchoice as a hook and tack \boldmath and \unboldmath onto it that way using a formula of \edef and \expandafters that I've yet to grasp/memorize. See How to add a hook to a macro.

In absence of hooking, I would say

\renewcommand{\choiceshook}{%
  \ifprintanswers
    \let\oldchoice\choice
    \let\oldcorrectchoice\correctchoice
    \def\choice{\unboldmath\oldchoice}
    \def\correctchoice{\oldcorrectchoice\boldmath}
  \fi}

\noprintanswers:

\printanswers:

You already seem to have answers that will work, but the exam class provides a hook for customizing the correct choices. If you give the command:

\CorrectChoiceEmphasis{\bfseries\boldmath}

then you'll get what you want. The default behavior was created with the command here:

\CorrectChoiceEmphasis{\bfseries}