How will the right answer fit in terms of support and conclusion?

Only the right answer will be a conclusion supported by the passage.

Highlight the main conclusion in the passage, if there is one:

There is no conclusion in the passage. The right answer will be the conclusion.

[BACKGROUND]. [BACKGROUND]. [SUPPORT].

You won’t necessarily know for sure which parts are support and which are just background until you start reading the answers. But because the answer is a conclusion, you’ll eliminate answers that bring in new stuff the passage doesn’t mention, and answers that use stronger wording than the passage uses.

Map the wording of the answers to the wording of the passage:

(A) …attempted to explain…

Very tricky. If they could actually explain it, this would be a totally reasonable conclusion, since the passage says not being able to explain it is a “problem”. But we can’t assume that’ll happen just because they attempt to explain it.

(B) …provide better explanations…

Whoa, no way we can make that comparison. All we know about “better established” theories is that they don’t try “to explain the nature and existence of gravity”. Which theories give “better” explanations is totally unknown.

(C) …has had at least some success in explaining why…

We have no way to conclude that. We only know it’s a “problem” that superstring theory can’t explain it. Maybe no theory can.

(D) A physical theory cannot be true…

Way too strong, there’s no support for saying the theory is impossible because it’s so hard to test.

(E) …is deficient if it cannot…

Love this. Clearly this maps to what the author said about superstring theory, since a “problem” is something that’s “deficient”. And it brings in absolutely nothing the passage didn’t talk about.

(E) is the correct answer.