When going through this question myself, I see the confusion because there is no outright endorsement for Gluck's view of poetry. However, it is important to note that answer choice D is "tacit endorsement" meaning implicit endorsement.
In that case, if D is going to be the correct answer as opposed to C (neutrality/indifference), then we need to find subtle clues pointing to the endorsement of Gluck's view.
We can actually find several clues, both within specific lines and using the overall presentation of ideas.
Line 14 - "Gluck embraces it with respect and admiration" While this may seem to be neutral because it seems like a fact. It is subtly positive. When speaking about people or positions that authors are not in favour of, there is typically less positive language used (e.g. embrace, respect, admiration).
Line 17 - Again the author notes Gluck's respect for the tradition.
Lines 25-28 - The author is agreeing with Gluck looking beyond gender distinctions.
Additionally, as far as structure goes, the author gives Gluck the last word when countering critics. Again, subtle but important.