In truth, the passenger who could not read this book saw nothing but all manner of pretty pictures in it painted by the sun and shaded by the clouds, whereas to the trained eye these were not pictures at all, but the grimmest and most dead-earnest of reading-matter.

the phrases “all manner of pretty pictures” and “most dead-earnest of reading-matter” emphasize the idea that

twain believes that the passengers are not as smart as he is.

twain wishes that the passengers could see the river the way that he does.

the passengers and twain perceive the river in very different ways.

the passengers are able to understand the river in a way that twain cannot

the passengers and Twain perceive the river in very different ways.

Explanation:

Right after it, Twain continues: "Now when I had mastered the language of this water and had come to know every trifling feature that bordered the great river as familiarly as I knew the letters of the alphabet, I had made a valuable acquisition."

He sees the river in a different way and much is to be told from what the river shows, it seems, but passengers are not able to see what he sees because they do not share the same knowledge.

the passengers and Twain perceive the river in very different ways.

Explanation:

the anecdote makes readers feel sorry for the man’s lack of heritage, then reminds readers that the man has an equally valuable heritage.

the answer is a hoped this