This question is asking you to find the principle that is underlying the argument. The best way to go about this is to look at the argument and see if you can determine what is behind it.
Here, the argument is telling us that some critics think its unfair for great art to be in city museums because the people in cities have access to other art. The argument then says that these critics are wrong because since there are few pieces of great art, they can't be distributed widely. And, it's important for great art to be seen with other works.
If you break down the argument, you see that the argument against the critics is essentially just that the critics idea that the art should be in smaller towns won't work either. The argument is finding holes in the critics idea.
Then, turn to the answer choices: (A) is incorrect. The argument does not focus on the numbers who are able to see the artwork. Rather, it points out two weaknesses in the critics argument, first that the art can't be distributed widely, and second that the art should be viewed in context.
(D) is correct. This answer choice explains that the argument is focused on saying that the critics don't have a better idea. Therefore, the underlying principle is that you shouldn't critique something if your suggested changes are not attainable.
