Printing solution of mcq using xsim

Starting with version 0.17 (2020/02/21) xsim has the exercise property solution which tells xsim to use the same environment body for the exercise and the solution. This is very convenient here. Using IfInsideSolutionTF then allows us to insert the checkmark where needed:

documentclass{scrartcl}
usepackage{xsim}[2020/02/21]

usepackage{enumitem,fontawesome,fmtcount,multicol}
newlist{choices}{itemize}{1}
setlist[choices]{label=faSquareO}

newcommand*correct{IfInsideSolutionTF{faCheckSquareO}{faSquareO}}

newcommand*choice{item}

DeclareExerciseProperty{choices}
DeclareExerciseProperty*{multiple}
DeclareExerciseEnvironmentTemplate{mc}
  {%
    UseExerciseTemplate{begin}{default}%
    IfExerciseBooleanPropertyTF{multiple}
      {Select one or more correct answers}
      {%
        GetExercisePropertyT{choices}
          {Select numberstringnum{#1} correct answerifnum#1>1 sfi.}%
      }%
    begin{choices}
  }
  {%
    end{choices}
    UseExerciseTemplate{end}{default}%
  }

DeclareExerciseType{mc}{
  exercise-env = multiplechoice ,
  solution-env = correctchoices ,
  exercise-name = Question ,
  solution-name = Solution ,
  exercise-template = mc ,
  solution-template = mc ,
  counter = exercise
}

xsimsetup{
  exercise/name = Question ,
  solution/name = Solution ,
  print-solutions/headings=false
}

begin{document}

begin{multicols}{2}
section{Questions}
begin{multiplechoice}[choices=1,solution]
  choice one
  choice[correct] two
  choice three
  choice four
end{multiplechoice}

begin{exercise}
  Answer this question on a separate sheet.  
end{exercise}

begin{multiplechoice}[multiple,solution]
  choice[correct] one
  choice two
  choice three
  choice[correct] four
end{multiplechoice}

begin{multiplechoice}[choices=2,solution]
  choice one
  choice[correct] two
  choice three
  choice[correct] four
end{multiplechoice}

columnbreak

section{Answers}
printsolutions
end{multicols}

end{document}