How to display the correct choice in red color in exam class?

The class provides the macro CorrectChoiceEmphasis for that (section 5.7 of the manual). The default definition is

CorrectChoiceEmphasis{bfseries}

so you can write

usepackage{xcolor}
CorrectChoiceEmphasis{color{red}bfseries}

for boldface, red text (or remove bfseries if you only want the text in red).

documentclass{exam}

usepackage{xcolor}
CorrectChoiceEmphasis{color{red}bfseries}

printanswers

begin{document}

begin{questions}

question Sample Question 1
begin{choices}
choice Option1
choice Option2
choice Option3
correctchoice Option4
end{choices}

question Sample Question 2
begin{choices}
choice Option1
choice Option2
    correctchoice Option3
choice Option4
end{choices}

end{questions}

end{document}