答案：A

考查从句。句意：什么使她如此高兴？她儿子通过了驾驶考试让她如此高兴。_____ her son passed the driving test补充完整是_____ her son passed the driving test made her so happy。所以_____ her son passed the driving test是主语从句，且从句句意完整，用That引导，B,C不能引导主从，而D只能做状语。