Answer:
Conditioned stimulus
Step-by-step 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, your friend's mother always bakes cookies when you are at their house. The cookies have the natural effect of making you salivate. Thus, the cookies would be the Unconditioned stimulus and the act of salivating would be the Unconditioned response. After many times, one day you start salivate as soon as you walk up the front steps to the house before smelling the cookies; we can see that you have paired the cookies with the house and therefore the house has become the conditioned stimulus that makes you salivate (conditioned response) as soon as you arrive.
Therefore, the reason for this is that the house has become a conditioned stimulus.