I believe your answer would be B ;). Hope this helps!
The correct answer is He withholds the fact that the falling horseman is the father of the shooter.
Throughout the story, the author yields a slow-paced description of the scene settings as a means to visually guide the reader. Moreover, they throw in clues such as the watchman's hesitation to shoot the horseman, as if the glance of the latter over the direction of the watchman caused him to react in that manner. This act may initially confuse the reader, but it isn't until the story's end that the horseman's identity is revealed, and so the climax of the story is explained and the surprise factor kicks in the reader.
The rest of the options are not as impactful since:
The watchman's conversation with his father seems ordinary at first, given the father's reaction to his son's desire to join the regiment. The revelation of the horseman's identity explains many of the phrases of this conversation, such as the father addresing the son as a traitor, meaning that both of the would fight on separate sides of the war. This is more fulfilling to the reader, rather than surprising.
The horse didn't bolt off the cliff until the watchman fired, but if it did before the shot, it wouldn't have been impactful to the story at any rate - considering the main surprise at the end.
