Answer:
The answer is D. Spontaneous recovery.
Explanation:
In conditioning theory, whether classical or operant conditioning, spontaneous recovery refers to the sudden reappearance of a behavior that was thought to be extinct. After the extinction of his conditioned fear of riding a bike, Alex suddenly experienced it two weeks later, this being an example of spontaneous recovery. This concept comes to show that extinction is not the same as unlearning, although he managed to eliminate his conditioned response, the behavior reapeared.