Answer: a. conditioned stimulus.
Explanation:
In the classical conditioning process, the conditioned stimulus which was a previously neutral stimulus, after becoming associated with the unconditioned stimulus generally initiates a trigger response.
For example, food is the unconditioned stimulus but the smell of food is the conditioned stimulus as it may generate the response of hunger in the subject.
According to the given situation, the difficult pop quiz is the unconditioned stimulus and closing the door is the conditioned stimulus. When the door closes the students can anticipate that Dr. Meyer is going to take a pop quiz.