Answer: an unconditional stimulus
Explanation:
An unconditional stimulus is the one which can generate a response in the organisms and the organism have not been introduced with the stimulus previously. The response of the stimulus is natural. It is the part of classical conditioning process. For example, when one smells the food, the person may feel hungry. The smell here is unconditioned stimulus.
According to the given situation, the dog is salivated due to the food which is here the unconditional stimulus which is responsible for generating a natural response in the dog.
