Which question best determines a story's mood?

How does the story make readers feel is what I got.

How does the story make the readers feel? is the correct answer.
Just think about what mood means, and that will you help you with answers such as these. None of these besides that are relevant to the question because they don't affect the mood whatsoever. So if the story makes you feel sad, then the mood is sad, therefore how you feel helps you determine the mood.
For example: if you were reading a chapter and the whole chapter was about the death of a girls mother, would you say that the chapter was sad?(the answer should be yes!) therefore how the chapter made you feel helped you figure out the mood!

The answer would be D. How does the story make readers feel?

The question that best determines a story's mood is How does the story make readers feel?
Mood in literature refers to the general atmosphere of a narrative, the emotional quality of a piece of art.