The best-supported inference based on the details in the passage is:
Harborough grew up in a city.
The passage states, "I was a city-bred youngster," which directly indicates that Harborough has a background in a city environment. This detail supports the inference that he grew up in a city.
While the other statements may have some basis in the text, they are less directly supported. For example, the statement about the Captain traveling widely is somewhat implied, but not directly confirmed in the passage. The mention of sailors not being good swimmers supports the idea that "most people who travel by sea cannot swim," but is not definitive. The statement about Harborough feeling superior is more of an interpretation of his feelings rather than explicitly stated in the text.