There is a verb tense issue here. "...when it is
known that humans made
stone tools" switches from present to past. To change tenses within a clause, you need an explicit change of time period or subject. Since (A) has neither, it is wrong.
(C) solves this problem by using only one verb ("are"). "To have made" is what's called a perfect infinitive, so we don't need to match tense since it's not conjugated.
To be honest though, I liked Ron's explanation
a lot. What he's saying is that (A) implies that 150,000 years ago, people knew that humans made stone tools, whereas 160,000 years ago, people didn't know that. This is clearly not the intended meaning of the sentence. Unfortunately though, that's just based on the definition of the word "when" - there's no larger rule here.
In option C, is it right to use "at which" for date. I found the usage a little awkward.