--When was it that Jack came back last night?杰克昨晚到底啥时候回来的？

--Well, it was before eight o'clock that he arrived home.他到家的时候没到八点。

It is/was+被强调部分+that...这个强调句型的一般疑问句写法是：

Is/Was it+被强调部分+that...

特殊疑问句写法：

特殊疑问词+is/was it that...

重点：这个强调句型去掉It is/was...that这个结构后剩下的部分能组成一个完整的句子

所以，上面放c选项进去句子直接就是错的，不可能选C。

—Did Jack come back early last night?

—Yes. It was not yet eight o'clock _____ he arrived home.

A. before            B. when            C. that            D. until

此题只能选 when。

It was not yet eight o'clock  when he arrived home.

when he arrived home，是时间状语从句。It was not yet eight o’ clock 是主句。

意思是：他到家时还不到八点钟。

其他答案都不成立。此句在逻辑上无法强调 not yet eight o'clock，况且之前缺少介词，所以排除 that。