After what seemed an endless wait, it was her turn to enter the personnel manager's office. 

此题只能用 what。选 that 仍然是错的，that 跟 it 没有根本的区别。

本句的关键问题在于：after 是介词，不是连词。what seemed an endless wait 是名词性从句，相当于一个名词，作 after 的介宾。

After what seemed an endless wait 意思等于：After an endless wait

after 不是连词，它在本句不能是连词。因为连词后面应该接一个“主谓句”，描述主语的动作行为。然而，如果用 it 或 that，那么，句子则是：

After it/that seemed an endless wait—— 一个系表结构，表达什么意思呢？

就像：After it was ten o'clock， after it was a long stay， 这些句子表达什么意义呢？