Correct Option (A)
The passage explicitly states, "He, however, soon fell asleep, being much tired with his long walk." This directly indicates that his exhaustion from the extensive walk was the primary reason for him falling asleep.
Incorrect Options:
Option (2): While the character felt lonely and "more lonely than he had ever felt before," the passage does not link this emotional state directly to the act of falling asleep. The stated cause was tiredness.
Option (3): The passage provides no information to suggest that the character had not slept for days. This is an assumption not supported by the given text.
Option (4): The text mentions that he "felt frightened at first." However, it then clarifies that he "soon fell asleep, being much tired," indicating that fear was not the cause of sleep; rather, it was a transient emotion preceding the onset of sleep due to fatigue.