Answer:
A. The exposition.
Step-by-step explanation:
The exposition of a story is the background information that a writer provides in his work. This allows the readers to have a sense of awareness about what the story is about, what the setting is, and other details that suggest or give information about what the story is moving to.
The given excerpt from Ambrose Bierce's A Horseman in the Sky describes the setting of the story. This setting creates the exposition of the story, thereby giving the background information about the setting before the real conflict arises. Thus, the correct answer is option A.