The girl turned red when she ____ to answer the question.A.asked B.was asking C.was asked D.has as

选c，原来的短语是ask sb to do sth让某人做某事, 被动形式是be asked to do sth。这句话的意思是当让她去回答这个问题时，她的脸变红了！

选C 从句缺少谓语 而且是被动语态

选C,be asked to do sth,被叫去做某事.
因为turn是过去式,所以be也要用过去式,即was

C.was asked
被叫起来回答。
所以用被动。