Dr. Graham exposes rats to a vanilla scent prior to receiving a food pellet in the left corner of their cage, but provides no food after exposure to a lemon scent. After several trials, upon smelling a vanilla scent, the rats wait at the far left corner of the cage regardless of whether a food pellet is present. However, they do not wait in the far left corner when exposed to the lemon scent. What is the unconditioned stimulus in the experiment?
a. food pellet
b. vanilla scent
c. lemon scent
d. left corner of cage

a. food pellet

Explanation:

In classical conditioning, an unconditioned stimulus is the stimulus that naturally elicits an unconditioned response. The unconditioned stimulus is usually paired with a neutral stimulus, and after pairing with a neutral stimulus, the neutral stimulus becomes a conditioned stimulus that elicits a conditioned response alone.

In the experiment described above in the question, the unconditioned stimulus is the food pellet, which naturally elicits the response of the rat to wait at the far left corner of the cage. The neutral stimulus which is paired with the food pellet is the vanilla scent, which now becomes the conditioned response, when paired alone.