Question

Which of the following would be a supporting detail when analyzing the theme in a story?

Answer 1

A twist in the plot is just that. It will move the story along, but it won't help the theme usually. 
A magical or supernatural character may or may not give any details at all. For example Pegasus, a flying horse, can fly and can rescue someone, but he normally does not speak. 
A short story does not have time for a summary. 

That leaves the second one which is the answer. 
Second one down <=========<

Answer 2

The correct answer is A) a quote that expresses the writer’s message.

The statement that would be a supporting detail when analyzing the theme of a story is a quote that expresses the writer’s message.

When you analyze the theme of any story you have to have in mind the opinion, the beliefs, the thoughts, the ideas, or the moment when the story was written. Remember that the theme is the main reason for the story. It's the main message. That is why a quote from the author could be very helpful when analyzing the theme of the story. This could sever as a reference that indicates reasons or motives that the author had when writing the story.