Final answer:
Observing a friend successfully quit smoking and then being able to quit yourself is an example of vicarious reinforcement, where the positive outcomes observed in another person's behavior increase the likelihood that you will mimic that behavior.
Step-by-step explanation:
Observing a friend's success at quitting smoking has helped you to quit a few months later, which demonstrates the process of vicarious reinforcement. This occurs when an observer, in this case you, sees a model, which is your friend, being rewarded for a behavior, such as quitting smoking, making it more likely for you to imitate the same behavior. The incentive comes from seeing the positive outcomes of your friend's actions, like improved health and the social approval they may have gained.