Marcel sat at a cafe eating the best croissant he had ever tasted. He begins frequenting the café to order the croissants. He hardly notices the jingling of the cash register just before the clerk hands him the croissant. Now every time he hears the same kind of jingling from another cash register, his mouth waters. Marcel's learning is an example of

a. operant conditioning.
b. latent learning.
c. conditioned taste aversion.
d. classical conditioning.
e. observational learning.

d