Answer: The correct answer is a) Conditioned Stimulus.
Explanation:
In classical conditioning a Conditioned Stimulus can be understood as an stimulus that was previously neutral gets associated with a unconditioned response eventually elicits a conditioned reponse.
In this particular case, Dr. Garcia closes the door everytime before he proceeds to do a pop-quiz to his students. Because the pop-quiz tend to elicit an unconditioned response of fear in students, the slamming of the door becomes associated with it.
As soon as the students hear the door, they anticipate the anxiety of the test.
In conclusion, closing the door has become a conditioned stimulus.