Answer: Extinction.
Explanation: Extinction in terms of classical conditioning is described as vanishing of the learned or used to behavior due to the absence of support or boost.In this condition ,the disappearance of unconditioned stimulus make the conditioned stimulus slow or stop the behavior that he/she has learned.
The example mentioned in the question about Bart is of extinction because he has stopped going to health club due to the absence of Abigail. Bart is conditioned for workout due to the presence of Abigail as unconditioned stimulus.