Answer:
Extinction
Step-by-step explanation:
In operant conditioning, extinction refers to the disappearance of the behavior that was learned by reinforcement because the reinforcement is now gone. In other words, the behavior stopped being reinforced.
In this example, Bart used to go to his health club every day because he almost always saw Abigail there. Abigail acted as a reinforcement for the behavior of Bart of going to the health club. However, for two weeks Abigail hasn't been there so the behavior of going to the health club stopped being reinforced (since Abigail, the reinforcement, disappeared). So now, the behavior disappeared because of the lack of reinforce.
Thus, this is a good example of the process of extinction.