Which of the following is an example of internal conflict?
character vs. society
character vs. character
character vs. self
character vs. nature

The correct answer is character vs. self.

If in a text a character has a conflict with him/herself, it usually means that it is internal - happening in their head only; they are unlikely to actually voice their conflicts with themselves out loud. On the other hand, if they have a conflict with other characters, with the entire society, or the nature itself, then those would be examples of external conflicts, happening outside the character's head.