选c.这是个强调句型.it was...that
在这里强调的是until引导的时间状语短语.还原一下是这样：
He didn't go back home until midnight after the long-lasting conference.
在强调until引导的短语或者从句时要注意把not提到until 前面一起放到被强调部分,也就是it is和that中间.
He didn't go to bed until he finished his homework.
变为强调句型：It was not until he finished his homework that he went to bed.
注意,把not提前后,went 用肯定形式了,也就是主句的谓语动词用肯定形式了.
祝学习愉快啊!