It always fits into the other hole, thus is the safest shape possible.

It doesn't matter (though the two answers given here are correct). The true (and unspoken) question is, "Are you curious enough to ask what the real answer is, despite the fact that the question has no relevance to the job?" IMHO, this is a test of curiosity. If the candidate gives their answer but doesn't ask a follow-up question like, "What's the right answer?" or "Is there a right answer?" they're not an innately curious person. Just my .02.