Which theory related to socialization believes that media content affects viewers' beliefs about the world and alters their behavior as a result?

The correct answer is the cultivation theory.

In sociology, the cultivation theory states that when people immerse themselves in the "television life" by spending significant amounts of time watching television, they are likely to believe that their own social reality is the same "reality' they see on television programs. According to this theory, heavy television watchers have altered views of reality and the world due to the influence of television, and this in turn affects their behavior.