I want to say it is a, however d does sound right to me also. I know b and c are not right because it just doesnt make sense with the sentence. Overall I would say a because it sounds the most right.
Hope you get the answer right! and let me know what the real answer was because I'm interested to see whether it was a or d.
Assuming that you're referring to the poem of the city of the yes and the city of the no, The poem uses an antithesis of Yes and No to represent openness and complete solitude