He must study hard,--- ---?(must 在这里是必须的意思) A.mustn't he B.n

mustn't he
反意疑问句要遵循前肯后否的规律.疑问句应该为“助动词+主语”,这里的助动词是must.而前为肯定,所以后用否定的mustn't.
而——Must I do it now?
——No,you needn't./Yes,you must.