Correct Answer: C
Explanation:
What sort of atmosphere would make Mars the only planet "whose surface details can be discerned from Earth?" You need a word that means transparent or thin for the blank. Vz'scous takes you in the wrong direction, so toss it. The next choice, ossified, makes no sense; toss that one, too. In contrast, rarefied works well, so hang onto it. Meanwhile, a copious atmosphere would definitely not be easy to see through, so cross out that choice. Meager fits nicely and agrees with rarefied, making those two the best answers.