External.
Because they aren’t fighting with themselves they are fighting an outside source.
Bro, if this is for a school assignment or something, you can't use someone else's life experience. It just won't work.
Answer:
Cheerful
Explanation:
Sullen means gloomy or in a bad mood. However, cheerful is the opposite (or an antonym) becuase it is an adjective that means to be happy and in a good mood.