(A) would be necessary for a conclusion that the tools were used by chimps rather than humans. If prehistoric humans carried their tools with them as they traveled (i.e. if we negate (A)), that would suggest the tools in the savanna could have been carried there by humans.

But that’s exactly what the argument is saying.

(B) should give you bad vibes because of its strong wording – “most of East Africa has been savanna for millions of years.”

(B) does gesture vaguely in a good direction. The correct answer, for instance, points out that what is savanna now may have been forest in the age of the chimps. (B) just overstates what’s necessary – the argument only needs this specific area where the tools were found to have been consistently savanna, not most of East Africa.

Like (A), (C) would be much more relevant if you switched chimps and humans. If prehistoric humans sometimes ventured into chimp territory (i.e. if we negate (C)), that would suggest that any tools found in chimp territory could be human.

We’re looking for the opposite – suggestions that tools found in human territory could be from chimps.

(D) negated: the area where the tools were found was a forest at the time the tools were in use.

When negated, (D) completely undercuts the basis on which the argument concludes chimps didn’t use these tools. Oh, chimps only lived in forests, you say? Well this area was a forest, son!

For the argument to succeed, then, the area in which the tools were found must not have been a forest back in the day.

This argument’s conclusion is about who used these tools. Whether chimps were also using other tools is irrelevant to that narrow conclusion.