Answer: B) To explain how experienced captains are able to “read” the river.
Step-by-step explanation: In the given passage we can see a metaphor (a figure of speech that consists in making a direct comparison between elements that aren't obviously related) that compares the river with a book, and how the passengers are unable to "read" it but the pilot (or experienced captains) can "read" the river as it was an italicized passage, which means that they are able to "read" it very easily.