Whiptails, the more speedier (A) of lizards, are remarkable creatures because (B) in thirteen of the known (C) species the females reproduce without (D) mating with males. No error (E) Could you please explain why a is the correct answer?

You should never put “more” in front of an adjective that already has an “-er” on the end. “Speedier” works by itself. “More speedier” is redundant.