Answer:
B) Is the story believable?
Step-by-step explanation:
All of them can be used in a realistic fiction (using all of them may make a better story) but I think the only one that is absolutely necessary is B, making the story realistic.
A realistic fiction story is a story written about events that did not happened but may as well have had happened, so the key into writing a realistic fiction story is the capability of the writer to make his or her characters, and the situations around them, as plausible as possible.
As I said, all of the choices can be used into the story writting, so, even if the second one is the only one absolutely necessary, all the others have the power to make your story a better and more interesting one.
Hope this helps!