Answer:
The answer is B. conditioned stimulus.
Step-by-step explanation:
In classical conditioning, a conditioned stimulus (in this case the sound of the door) is associated with an outcome (unconditioned stimulus), thus provoking a reaction (unconditioned response).
This was explained by Ivan Pavlov, who carried out an experiment in which he rang a bell every time he was going to feed a dog. Soon, the dog would start salivating at the sound of the bell.