He will ----- our class to sing at the school singing contes

是选C.这句话要表达的意思是“他代表我们班去参加学校的歌唱比赛.”
A选项是“发生”,当然不对；B选项是“支持”,stand up for才是代表的意思；representative是名词或形容词,代表的意思,如果改成He is the representative of our class.就对.