How does the mood of a story affect the reader?

it determines whether the conflict is internal or external.
it reveals the character’s motivation.
it makes the theme harder to figure out.
it affects the reader’s interpretation of events.

I think the correct answer from the choices listed above is the last option. The mood of a story affects the reader by influencing the  reader’s interpretation of events. It would stimulate the reader's mind to think. Hope this answers the question.

The mood of a story affects the reader’s interpretation of events.

The mood is the part of the narrative structure in a literature. It envelops the reader by creating the emotional atmosphere. To provide the feeling of the narrative is provided in order to affect the reader psychologically and emotionally. It can be created through theme, diction, and setting.  

For instance, Charles Dickens in his novel “Pickwick Papers” creates a peaceful mood by:

“The river, reflecting the clear blue of the sky, glistened and sparkled as it flowed noiselessly on.”

It helps the reader to interpret the event and feel it emotionally and psychologically.

I believe your answer would be option A not entirely sure but it's not B, C, or E so that leaves A or D hope this helps