The correct answer is UCS
Explanation: Unconditioned stimulus (UCS) is the one that unconditionally provokes a natural and automatic response. For example, when you smell one of your favorite foods, you may immediately feel very hungry and salivating. In this example, the smell of food is the unconditioned stimulus.
Unconditioned response (UCR) is the response that occurs naturally in reaction to the unconditioned stimulus. For example, if the smell of food is the unconditioned stimulus, the feeling of hunger in response to the smell of food is the unconditioned response.
The feeling of hunger in response to the smell is an unconditioned response, and a whistle sound is the conditioned stimulus.
The conditioned response (CR) is the learned response to the previously neutral stimulus.
