Explanation

Creating a filter: we can stop early in the original sentence, because we come across the words "there being," an unnecessary gerund. It can simply be dropped; we are talking about Fama's "theory of efficient financial markets." Let's eliminate (A) and move on to the answer choices.

Applying the filter: Choice (B) distorts the intended meaning, because it doesn't make clear that having average returns that can't be exceeded is the definition of what makes a market efficient; with the word "whose," that fact is coming across as additional information about the markets. And the modifier "given widely available information" is floating in space, rather than as a condition for not exceeding the average returns. So (B) is out. Choice (C) is better, so we can come back to (C). In choice (D), the use of "which" would initiative a nonrestrictive clause; in other words, it implies that the theory is uniquely defined, in which case the "which" phrase must be set off by commas. It seems to indicate that Fama has only one theory. The phrase "there can be no investor" also expresses the idea inaccurately, because Fama is really talking about what existing investors are able to do, not what kinds of investors are allowed to exist by the rules of physics or something. Choice (D) is out. Choice (E) has another "which" clause that needs commas, and (E) also distorts the meaning by taking the main point -- the fact that the theory is still respected -- and presenting it as a minor fact about the theory. And it presents the definition as the main point. So (E) is out. The correct answer is (C).