语法高手请帮忙

A
这是一种虚拟语气的用法

一般是if引导的

主句用might/would/should have done sth,从句用had done sth
表示“要不是...就...”

此句意思是“要不是因为她摔断了腿，她就通过考试了”，

之所以说虚拟，是因为表述的事情都是假设的（假的，与事实相背的，“虚的”），这里即是表达她没有通过考试

一般结构为“if sb had done sth1,sb would/might have done sth2”，这里是一种倒装结构，省略了if ,即变成“had sb(it作为形式主语也可) done sth1,sb would/might have done sth2”

类似的句子还有：

if i had bought the car,i would have lost a big money.

had he brought her back,she might have been caught in the hurricane.

选A吧
整句应该是虚拟语气吧，我似乎从没接触过B的说法，不太清楚。
主句“she might have passed the exam”已经是过去时了，那从句就应该用过去的过去咯。
纯属个人意见~呵呵