We are looking for the answer choice that works as the principle the author uses to counter the critics' claim.
Critics claim that "it is unfair that so many great works of art are housed in huge metropolitan museums, since the populations served by these museums already have access to a wide variety of important artwork." The author explains that the criticism is unwarranted because the wider distribution of these limited number of masterpieces would be impractical, and "if a masterpiece is to be fully appreciated, it must be seen alongside other works that provide a social and historical context for it."
Answer choice (D) is the principle that the author appeals to in countering the critics' claim.
"For it to be reasonable to criticize an arrangement as unfair, there must be a more equitable arrangement that is practically attainable." Let's diagram:
RCUF = Reasonable to criticize an arrangement as unfair
MEAPA = there is a more equitable arrangement that is practically attainable
RCUF ==> MEAPA Not MEAPA ==> Not RCUF
The author explains that wider distribution of the art is impractical, i.e. there is not a more equitable arrangement that is practically attainable. Thus, the contrapositive of answer choice (D) is invoked, Not MEAPA ==> Not RCUF.
So, answer choice (D) is the principle that the authors uses to conclude that criticizing the arrangement of great works being housed in huge metropolitan museums as unfair is unwarranted.
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