Final answer:
Jimmy's outsmarting of Bob through a disguised arrest is an example of situational irony, a literary device that involves a contrast between expectation and reality.
Step-by-step explanation:
Jimmy's outsmarting of Bob at the end of "After Twenty Years" is an example of the literary device known as irony, specifically situational irony. Situational irony occurs when there is a discrepancy between what is expected to happen and what actually happens. In the story, Bob expects to meet his old friend Jimmy, but instead, he encounters a police officer who he does not realize is Jimmy in disguise. It's only after the officer departs and a plainclothes policeman approaches Bob that the truth is revealed: Jimmy has arranged for Bob's arrest, as Jimmy is now a policeman upholding the law. The impact of this twist lies in the reversal of expectations and the stark contrast between the loyalty of old friendship and Jimmy's duty as a law enforcer.