I am still a little bit confused.
the conclusion is "Therefore, ancestors of modern humans lived in Western Asia between 2 million and one and a half million years ago." (nothing in the conclusion specifically mentions that humans lived during the time of the lake's existence)
This conclusion is in the form of a max and min range of possible dates. If anything falls outside of these dates, the conclusion is invalid, in my opinion. (ANT on Answer C) So it is of importance if the lava that lay under the lake bottom contained human fossil remains, as it shows that humans lived prior to 2 million years ago, which is a violation of the range expressed in the conclusion.
Answer E follows the same logic, it presents the statement that bones were present after 1.5 million years, again a violation of the date range in the conclusion.
How is E better than C?