When Maurice's father walks through the door after the end of a long day at work, Maurice jumps up, claps his hands, and runs to see Daddy. Lately, though, he has been dropping his toys and getting excited when he hears his father's keys in the lock. The fact that the response has changed stimuli is a demonstration of:.
a. operant conditioning
b. habituation.
c. the orienting response.
d. classical conditioning.

d. classical conditioning.

Explanation:

Classical  conditioning is a form of learning that pairs two different stimuli repeatedly to create a new behavior.

Originally, one stimulus triggers a natural response by itself (this is a unconditioned stimulus and the response is the unconditioned response), however, this stimulus is presented accompanied by another one who doesn't trigger a response by itself and, after some time, this stimulus that was a neutral stimulus, starts triggering the response too (and it becomes a conditioned stimulus and a conditioned response). In other words, the original response happens in presence of the other stimulus.

In this example, the presence of his dad makes Maurice excited (this is a natural response), however, Maurice has associated the sound of the keys in the lock to the fact that his dad has arrived, so now, the only sound of the keys in the lock makes Maurice excited (and so, this answer of excitement is appearing in the presence of the originally neutral stimulus that was the sound of the keys). Therefore, the fact that the response has changed stimuli is a demonstration of Classical conditioning.