Answer: Because Po believed in himself.
Explanation: Independent clauses are things that express a complete thought. He defeated Tai Lung is a complete thought.
Because Po believed in himself, is the dependant clause. "Because Po believed in himself" is not a complete thought.
Words like "because, therefore, unless, etc." are all used as the beginning of a dependent clause.
Independent clauses must have a subject and a verb. The verb is defeated, and he (or Tai Lung, I'm not so sure about this) is the subject.
