Answer: I would contend that the right answer is the C) that he will be the only observer of the horseman's fall.
Explanation: Just to elaborate a little on the answer, it can be added that by making his way "with aimless feet" and "pushing his exploration further," the officer had the potential chance of witnessing something unexpected and adventurous. And this he did, when he saw the horseman in the sky. He did hurt himself in his shin when he saw the man on horseback riding through the air, but not seriously, and this description of his spirit of adventure does not necessarily suggest that he will lie to the commander.