Explanation

Creating a filter: When we read the sentence, we hit a flag. We have suspicious pronoun, "they." "They" is a pronoun with a vague reference. As the sentence is written, "they" refers to the Flesch-Kincaid grade levels, but that's not what the imaginary author of this sentence is trying to express. The correct answer must fix this error, so we have an expectation to apply to the answer choices as a filter.

Applying the filter: Choice (A) is the only answer choice with the "they" problem, but (B) and (D) have an "it" problem. "It," like "they," is a pronoun without a clear reference. Actually, (E) has it too, and the word "when" is also dubious, because we are not really describing something that is related to time, but rather to the logical characteristics of a passage of text that is being graded. So (C) must be right. We plug it back in. It's elegant; this is what the imaginary author of the sentence was trying to say. The correct answer is (C).