Final answer:
In Pavlov's experiment, the bell is the conditioned stimulus that elicits salivation, which is the conditioned response after the dog has been trained to associate the bell with food. So, the correct answer is option a.
Step-by-step explanation:
In Pavlov's experiment with classical conditioning, the dog would salivate when it heard the ringing of the bell because it had learned to associate the bell with food. The bell in this context is known as the conditioned stimulus because it is a previously neutral stimulus that, after being associated with the unconditioned stimulus (food), now triggers a conditioned response (salivation).
Initially, the bell had no effect on the dog's behavior, but after repeated pairings with the unconditioned stimulus (food), the bell alone could elicit salivation. The conditioned response (salivation to the bell) differs from the unconditioned response (salivation to the food) in that it was learned rather than innate. So, the correct answer is option a.