assuming that C2 contains a check box....
you can test whether it is checked by using the IF() function like:
=IF(C2 = <CELL WITH ANSWER>, "CORRECT", "INCORRECT")
<CELL WITH ANSWER> is the cell where the correct answer resides as a "TRUE" or "FALSE"
If you provide a framwork of what you are trying to do I can provide more specific information
I would go a little further. These are checkboxes, not radio buttons, so the test taker could check more than one box by mistake (or on purpose, thus guaranteeing they get the right answer). I think it needs to make sure only one box is selected.
If the possible answers are in C2, C3, and C4:
=IF(COUNTIF(C2:C4,TRUE)>1,"more than one answer selected",IF(C2,"correct","incorrect"))
You can sum up all the correct answers with
=COUNTIF(<whatever column has the formula above for all the questions> ,"correct")
EDIT: Actually, this idea isn't ideal either since it gives the answer away but maybe it'll get you a little further to the best solution. I do think you need a box somewhere that tells the test taker if they have selected more than one answer and maybe it should tell them if they have yet to select an answer. And your scoring formula needs to know if they slected more than one answer.