As written, this sentence contains a parallelism error. When a comparison begins with the more..., it must be followed by a parallel comparative the ___. For example, you would say, The more, the merrier. Here, the sentence reads, "the more he reminded ... their willingness ... increased," which is not parallel.

A scan of the choices shows a 3-2 split, with (A), (B), and (C) beginning with "their willingness," while (D) and (E) begin with "the greater." Since only "the greater" is parallel to "the more," (A), (B), and (C) can be eliminated.

(E) does begin with a parallel comparative, "the greater." (E) introduces a couple of new errors, however. First, "advanced preparation" seems to describe the level of preparation, perhaps as opposed to basic preparation. The original sentence intends to describe the timing of the preparation - preparation done before the recital. (E), therefore, changes the meaning of the sentence. Additionally, (E) contains a redundancy, with "the greater ... increased." It's sufficient to say "the more ... the greater." The "increased" at the end is unnecessary. For these reasons, eliminate (E).

(D) corrects the parallelism problem without introducing any new errors and is, therefore, the correct answer.