The author is the origin of the text. And when you are examining the source it is possible that you could look for the author's purpose.
Sentence A is correctly punctuated.
For the other answer choices, a question mark shouldn't be placed.
The answer is A: Its set against a historical backdrop
If I understand the question correctly, the negative is the "opposite," so here you could write a phrase with the opposite of each word:
The moon sets to the west.
