C vs D vs E

Good question, main point questions are hard, and often the right answer may not jump off the page as correct, but rather, it will just be something that we are unable to poke any holes in. Off the bat, C seems like a good summary of the passage. I can't really attack it well, and since the author wasn't taking a strong argumentative position, C seems like a good option. D somewhat misstates the passage. While Sibley did introduce new methods, I don't know if they really call into question other methods. Additionally, the entire passage has an emphasis on the importance of the question of species, and the ongoing uncertainty of defining, both of which are missing here.

E suffers from the same issue of D, the main point of the passage wasn't solely to look at Sibleys work.