Explanation

Someone worried about having to analyze an esoteric poem would be delighted to get one that was clear or easily understood, so something like clarity would make sense in the blank.

Both ambiguity and opacity are nearly the opposite of what you’re looking for, so eliminate choices (B) and (F). Neither cadency, which means rhythm, nor melancholy, which means sadness, is supported by the sentence, so eliminate choices (A) and (D). Both cogency and lucidity can mean clarity, so choices (C) and (E) give you appropriate, equivalent sentences.

Hence C and E is correct!