-What made her parents so angry?-_That____ she had failed in the examination.为什么选that

答句是真正回答的主语，整个句子作主语，成为主语从句，主语从句本身是陈述句，要用that来引导。
完整的句子是That she had failed in the examination made her parents so angry.

这是一个省略句,完整的句子应该是:It's because that she had failed in the examination.

楼上的回答错了~
看句子
问题中说What made her parents so angry?
what指代的是令她父母很生气的事~是这句话的主语
所以~回答的就是这个what的内容,即"他考试不及格"
因为这是一个句子~所以要用主语从句~此时要有引导词That
完整的句子是
That she had failed in the examination made her parents so angry
如果要句子更符合英语习惯应该用强调句了
It was that she had failed in the examination that made her parents so angry