I believe the question in the example is unfair, and if you brought that up to the teacher you would look like you knew even more theory than what you have studied in class. As explained here on this thread, the idea of a diminished unison is quite debatable and as such is not a suitable question for a student to answer. If I faced your situation, I would leave the answer blank and go talk with the teacher. 

In my opinion, a diminished unison is like dividing by zero. An answer cannot be found. The idea of an interval is the distance between two notes. We can adjust those distances and make them smaller or larger. No matter what is done to a unison, it is made larger. C to Cb is an augmented unison. C to C# is an augmented unison.