Final answer:
In an advertisement that uses classical conditioning to persuade, the product or service being promoted is usually a c. positive reinforcer.
Step-by-step explanation:
In classical conditioning, an advertisement that uses classical conditioning to persuade usually associates the product or service being promoted with a conditioned stimulus (CS).
The CS is a neutral stimulus that becomes associated with a positive reinforcer, such as a desired feeling or outcome, through repeated pairings with an unconditioned stimulus (UCS).
The UCS is typically something that naturally elicits a positive response, like pleasure or satisfaction. Therefore, the correct option is c. positive reinforcer.