Final answer:
The correct answer is Option C, unconditioned stimulus (UCS), which naturally elicits a reflexive response such as salivation in Pavlov's dog experiments, unlike a discriminative stimulus that does not originally elicit a response.
Step-by-step explanation:
An SD, or discriminative stimulus, does not cause or elicit the appearance of a response the way an unconditioned stimulus (UCS) does. Therefore, the correct answer to the question is Option C, unconditioned stimulus (UCS).
In thecontext of classical conditioning, a neutral stimulus is something that does not naturally elicit a response in an organism. For example, in Pavlov's experiments, a tone (the neutral stimulus) did not cause the dogs to salivate until it was associated with meat powder. The meat powder represents an unconditioned stimulus, because it naturally triggers a reflexive response, which is known as an unconditioned response (UCR) - in this case, salivation.