Final answer:
The best summary of the excerpt from 'Robinson Crusoe' is that the narrator climbs a hill and finds he is on an isolated island.
Step-by-step explanation:
From the given excerpt of 'Robinson Crusoe' by Daniel Defoe, the sentence that best summarizes the passage is option A. The narrator, who is also the protagonist of the story, ascends a steep hill with effort and discovers the isolation of his situation. Despite his initial uncertainty about his location, he confirms that he is indeed on an island after reaching the hilltop and observing his surroundings. This discovery brings him great affliction as he realizes his solitude.