Scientists have dated sharp-edged flakes of stone found in the fine-grained sediments of a dry riverbed in the Afar region of Ethiopia to between 2.52 and 2.60 million years ago, pushing back by more than 150,000 years the earliest date when it is known that humans made stone tools.

1. when it is known that humans made
2. at which it is known that humans had made
3. at which humans are known to have made
4. that humans are known to be making
5. of humans who were known to make

I selected C. But the answer key shows A. Any comments?

我怎么觉得是D呢?

A: earliest date when it is known that humans made 表示,最早知道人类做了石器这个事实的那天, 而正确表示应是最早做石器的那天.

C: date + on which

我倾向A，主要是觉得其它的错误太明显。BC中的介词AT应该为ON，D中的不定式进行时都是典型的错误，而E更是语意错了。A中的 it is known that是插入语，这样就比较好理解了吧，这样就成了简单的时间状语从句了，不知道对不对。

As I said before, A has an obvious logic error. "when" can either qualify "known" or qualify "made".

This is what I found from another

总算找到原文重现了;呵呵

This so-called Oldowan phase of toolmaking ... years ago, pushing back by more than 150000 years the known date at which humans were making stone tools 可能是英文习惯用法:THE DATE AT WHICH

With the knowledge that "date at which" is correct, I prefer Answer C.