Answer:
The answer is the food.
Explanation:
In classical conditioning, a conditioned stimulus (the sound of the bell) is associated with an unconditioned stimulus (the food). This produces a reaction known as conditioned response (the salivation).
Unconditioned stimuli are those which naturally produce a response. Salivating when tasting food is only logical: it has not been conditioned. These stimuli can be paired with an unrelated stimulus (e.g. a sound) in order to condition the response.