In the United States, most people understand that they are expected to form a line and wait for their turn, even if they have never been explicitly instructed to do so. That is an example of what type of rule

It's called an implicit rule

Explanation:

An implicit norm or idea is a common understanding or communication among people that is not explicitly mentioned or expressed but is recognized by the moral compass.

It is not explicitly expressed or stated, but civic sense understands it.

So like the example says, those who understand that they must form a line and wait their turn are following an implicit rule.

answer; the correct answer is (travel horizontally);

its kind of blury sorry