Mark Twain wants to emphasize that “Travel helps people understand the world around them.” since it opens our mind to new worlds, new people and their ways of living.
Option 2 is incorrect since traveling in itself is learning experience no matter if you are educated or not.
Option 3 is incorrect as Twain means exactly the opposite, that travel opens your mind, it doesn't close it.
Option 4 is also incorrect since Twain does not refer to the costs of traveling and he does say that it is useful to most people.