The clearest reason (A) is wrong is that the passage does not present evidence in opposition to the alternative theory, nor does the author argue in favor of rejecting the alternative theory. If “theory” in (A) is supposed to refer to the traditional theory that electrical impulses travel electrically, (A) is still wrong because there’s no evidence offered in support of that theory.

(B) mentions a “theory” and an “alternative theory.” The “theory” must refer to the traditional belief that electrical impulses travel electrically, because the “alternative” would be what the passage calls the “alternative theory” (that electrical impulses are transmitted chemically). Under this interpretation, (B) goes wrong because there’s no evidence presented in support of the “theory” — there's only evidence presented in support of the “alternative theory.”

This is the best answer. In P1, we get an explanation of a theory (the alternative theory that electrical impulses are transmitted chemically). In P2, we get an obstacle to the theory’s acceptance (scientists couldn’t imagine how chemicals could influence the flow of ions). P3 provides an explanation that helps overcome the obstacle (interaction between neurotransmitters, binding sites, and ion channels). P4 discusses a further implication (we might develop drugs for treating brain disorders).

The author never argues that an obstacle is “insurmountable” or cannot be overcome. The author also doesn’t argue that the alternative theory should be rejected or discuss the implications of rejecting the alternative theory.

If “theory” in (E) refers to the traditional theory (electrical impulses transmitted electrically), then (E) is wrong because the author doesn’t describe how this theory gained acceptance. If “theory” refers to the “alternative theory,” then (E) is wrong because the author never presents information that challenges the alternative theory, nor does she modify the alternative theory.