A denouement is the final part of a play, movie, or narrative in which the strands of the plot are drawn together and matters are explained or resolved.
And based off the answers...
A. By returning to the original location, Bierce ends the story by contrasting Druse's actions to the beginning of the story.
B. The description of the Federal sergeant crawling to Druse allows Bierce to show how the soldiers must remain hidden.
C. Describing Druse's location and feelings allows Bierce to enhance how monstrous Druse feels at the end of the story.
D. Through the setting, Bierce is able to explain the climax and resolve Druse's feelings about killing his father.
I believe that the correct answer is D.