I presume you are referring to an online exam in which there is no professor to speak to, and no margin to handwrite an objection.
So, what I would do in your shoes is that I would pick the answer that most closely matches "undefined" (something like "prohibited" or "crash") and if none of these are available then I would just pick a guess. Then, I would wait until my exam gets graded, and if my answer to this question is flagged as wrong, I would take it to the dean.
Start looking for the dean's email from now.
The most famous instance of undefined behavior in C is this:
int somefunction( int x, int y );
int a = 5;
int result = somefunction( a, a++ );
The question is "what is wrong with the above call?"
I came across this one in an interview once, but it was just for bonus points, they did not really expect the candidate to know. The answer is undefined behavior because C does not guarantee to you that 'a++' will be evaluated after 'a'. So,
somefunction may be called with
x=5, y=6 or it may be called with
x=5, y=5 or
x=6, y=6. (
or a whale and a bowl of petunias might fall from the sky <-- EDIT: no, this is not a possibility.)