Answer: 4. Food
Explanation:
Classical conditioning is a learning process in which a biologically stimulus is supported by the neutral stimulus or primary reinforces to trigger the response in the subject. The unconditioned stimulus is always paired with the conditioned stimulus to elicit a response in the subject.
The food is a primary reinforcer and it is not a conditioned reinforcer. The primary reinforcers does not require any other stimuli for triggering a response in the subject. As the food is the basic need of the living organisms and required for survival thus the living being will eat food and are dependent upon food thus the food consumption process does not require any other stimuli.
