A pigeon is consistently reinforced with food for pecking a key after seeing an image of a human face,but not reinforced for pecking after seeing other images.By signaling that a pecking response will be reinforced,the image of a human face is a(n)

A) unconditioned stimulus.
B) partial reinforcement.
C) discriminative stimulus.
D) primary reinforcer.

The image of a human face is a discriminative stimulus because it signals the availability of reinforcement for the pecking response. This is different from an unconditioned stimulus, which is a stimulus that naturally elicits a response without any conditioning or learning involved. Partial reinforcement refers to a situation where reinforcement is not given for every correct response, which is not the case here as reinforcement is consistently given for the pecking response after seeing a human face. Primary reinforcer refers to a stimulus that is inherently reinforcing, such as food or water, which is not the case here as the human face needs to be associated with the delivery of food to become reinforcing.