Because " . . . the purpose of art is to cause experts to debate ideas, including ideas about what constitutes are itself" and "the edifice has caused experts to debate what constitutes art itself."
This is a Strengthen with Sufficient Premise question so we are looking for the answer choice that 100% guarantees the conclusion of this argument.
(A) would be an incorrect reversal.
"Nothing qualifies as art unless it causes debate among experts."
QA ==> CDE
We know "CDE" but we cannot go backwards to conclude "QA". So (A) would be eliminated.
(D), on the other hand, would tie the premise, i.e. fulfilling the purpose of art, to the conclusion, i.e. qualifying as art.
FPA ==> QA
We know that this edifice fulfills the purpose of art because it has caused experts to debate ideas, which would invoke the sufficient condition in (D) and allow us to conclude with 100% certainty that the edifice qualifies as art (i.e. the necessary condition).
Therefore, (D) would be the correct answer.
Hope that helps! Please let us know if you have any other questions.