Question

Darkness settles on roofs and walls,

Answer 1

I believe the answer is: Mystery

A sense of mystery tend to be created when the situation presented in the writing seem uncertain and difficult to explain.

The use of the word darkness in both lines depict something that couldn't be easily seen/noticed and yet the usage of the words 'settles' and 'calls' means that the speaker is sure that something exist in the darkness.

Answer 2

The answer is C, mystery. Darkness is significant of the unknown. D would be a reasonable choice, but there is no proof that darkness signifies despair in this case.