The correct option is C when it was.
Following the syntax of assertive sentences, option C is the correct one, having the interrogative adverb of time/ question word followed by the pronoun and then the verb. Taking into context the topic of discussion, it would be safe to assume that the students would be wondering about the time frame of said activity and not the reason, which therefore, makes option A incorrect. Options B and D end in questions. The statement have started as an assertive sentence, Options B and D would be incompatible in this context. Thus, option C is the most appropriate answer.