An event in a story that's exactly the reverse of what was expected to happen is an example of:

A situational irony

B dramatic sarcasm

C character action

D subtle conflict

An event in a story that's exactly the reverse of what was expected to happen is an example of situational irony. Since the event is a situation and irony means "the expression of one's meaning by using language that normally signifies the opposite, typically for humorous effect or empathetic effect".

Definition of irony from google.