The underlined portion of the original sentence incorrectly connects "The reason..." with "...is because..." The correct usage is "The reason...is that..." Additionally, the word "that" after tryptophan is used incorrectly. “That” is used to add information necessary to identify the noun that precedes it. If the information is not necessary, use "which." Here, tryptophan was already identified by name, so the additional information is unnecessary. “That” should therefore be replaced with “which.”
Scan and Group the Answer Choices:
Apart from whether they contain the errors noticed above, (A), (D), and (E) have the verb “improve,” while (B) and (C) have “improves.” The subject of that verb is "meats," a plural noun. Plural nouns need plural verbs, so "improve" is the correct form ("meats...improve").
Eliminate Wrong Answer Choices:
Interestingly, only (A) uses "that" after "tryptophan." All of the other choices correctly change "that" to the correct "which."
Only (C) and (E) correctly use "The reason...is that..." (A), (B), and (D) can therefore be eliminated. (B) also uses the the wrong verb form of "improves." (D) also introduces two new errors: the pronoun “they” is ambiguous, and the word “like” is used incorrectly to compare verbs.
(C) correctly uses "The reason...is that...," but contains the wrong verb form of "improves" and can thus be eliminated.
(E) is correct, as it fixes both noticed errors and uses the correct form of "improve"
TAKEAWAY: When you have a long sentence with a large portion underlined, look out for multiple errors and work first with whichever error you prefer. Don't forget that choices can introduce new errors as well.