Explanation

Creating a filter: On our read of the prompt, "that since" may sound a little funny. However, "that" introduces the statement that is the belief of the scientists, according to proper usage. And the statement just happens to begin with the word "since," which introduces a dependent clause. Moreover, all the answer choices have the phrase "that since." Supposing that we don't see any other error, we can move to the answer choices without a filter and search for objective defects.

Finding objective defects: All the answers are starting out the same, but we see differences at the ends of the lines of each answer choices. "And," "should," "did." To find objective defects, we can first compare the answer choices and look for differences.

Choice (B) is flawed because of the "and" at the end: it changes the meaning of the sentence. The scientists are drawing a conclusion in this sentence, and that conclusion is the last four words of the sentence, a clause all on its own, and since it's the sole conclusion, what precedes it is evidence for the conclusion, and it shouldn't be preceded by an "and."

Choice (C) is out, because it distorts the intended meaning by replacing a finding with an expectation or moral view indicated by "should."

Choice (D) is flawed in a similar way to (C): "did outperform" does not need a "did." The word "did" seems to imply a contrast with a prior opinion or belief, but there are no hints in the sentence of such an intended meaning.

That leaves us with choices (A) and (E), which are similar.Comparing them, we find that (A) has an error that we hadn't noticed at first. It has no main verb! The phrase starting with "concluding" is a dependent clause and can be logically extracted from the sentence, leaving "scientists caffeine enhances" and there is no verb in the sentence. So (A) is wrong; it failed to pass our first universal test, the test of the subject and verb of the independent clause. The correct answer is (E).