Dr. Meyer is known for his difficult pop quizzes. Immediately before he announces a pop quiz to his students, he walks to the classroom door and closes it. His students soon learn to anticipate a pop quiz whenever Dr. Meyer closes the classroom door. In the context of classical conditioning, closing the door has become a(n)

The answer is conditioned stimulus.

Explanation:

In classical conditioning, a conditioned stimulus (the sound of the door closing) is associated with an unrelated consequence, known as unconditioned stimulus (the pop quizzes). This produces a reaction known as conditioned response.

It's important to recognise that any neutral stimulus can become conditioned. In the example, instead of closing the door, Dr. Meyer could choose to slam his books on the desk. When paired with the quizzes, this would create the same response.