That's a very strange question for a nursing instructor to be asking of a student, in my opinion. The reason I say that is because nursing diagnoses are based upon the symptoms that a patient has. The problem with bipolar disorder is that the patient can either be manic or depressive, so it seems to me to be a 50/50 toss up.
I don't, as a rule, list nursing diagnoses without proper symptoms to support them, but since this is such as odd request I'll list the likely nursing diagnoses assuming the patient has the classic symptoms of the disorder. I'll leave it to you to figure out which one might be the MOST common.