Funny 5th option in multiple choice exam?
That is hard for us to determine. This could be a legitimate way to help students relax during the exam. The fact that the professor includes this at a fixed spot at the end of each question, that this seems to be an established tradition with students expecting this and being encouraged to participate in it, points in that direction.
On the other hand attempts at humor directed towards a large number of people in a context where you cannot easily defuse misunderstandings is risky.
So based on the information you have given us, it could be either.