Answer:
Unconditioned stimulus
Explanation:
Classical conditioning is a form of learning by associating stimulus to responses and by pairing them in order to learn.
In classical conditioning, first we have a stimulus that triggers a natural response by itself. This stimulus is called the unconditioned stimulus (US) and the natural response triggered is the unconditioned response (UR).
Later, we start presenting a stimulus that doesn't trigger any response by itself and we pair this stimulus with the unconditioned stimulus. This new stimulus is called the conditioned stimulus (CS), soon, after some repetitions, the response will start to show up in presence of the CS, and now the response is called conditioned response (CR).
In this example, Colin was spanked on several occasions for spilling his milk at a restaurant so he became afraid to go to the restaurant. The spanking is an stimulus that triggers a natural response of being afraid. Therefore, the spanking would be an unconditioned stimulus for Colin's fear.
