KAPLAN OFFICIAL EXPLANATION
Read the Original Sentence Carefully, Looking for Errors:
Generally, use of the passive voice is incorrect on the GMAT, and this sentence commits this error with the phrase "creative processes that were engaged in by ...." Look for an answer choice that actively tells what T.S. Eliot did, rather than what was done by him.
Scan and Group the Answer Choices:
(A), (B), and (D) keep the passive "that were engaged in," while (C) and (E) each use a more active construction.
Eliminate Wrong Answer Choices:
(A), (B), and (D) can be eliminated for using the passive voice.
(C) doesn't use the passive voice, but it awkwardly changes the meaning of the sentence. In the original sentence, Eliot's poems provided evidence about his creative processes. In (C), Eliot provides evidence about the poems' creative processes. This doesn't make any sense.
(E) provides a more active construction ("in which ... engaged") and doesn't introduce any new errors, so it is correct.
TAKEAWAY: If an answer choice provides an active voice alternative to a sentence in passive voice, without introducing any errors of grammar or style, it is correct.