Answer: D. must, be.
Explanation: an auxiliary verb is a verb used to add functional or grammatical meaning to the clause, for example to express tense, mood, voice, or to form a negative or a question. Auxiliary verbs usually accompany a main verb which is the one that expresses the action. In the given sentence, the main verb is "pass", in the participle form "passed" and the auxiliary verbs are "must" and "be."
