In classical conditioning, what is the unlearned, naturally occurring response to the unconditioned stimulus (US), such as salivation, when food is in the mouth?

The answer unconditioned response.

Explanation:

Unconditioned response is a natural reaction we show when presented with an unconditioned stimulus. It is an automatic, unlearned response, a reflex. Salivation, as mentioned in the question, is a natural response from our body when we have food in out mouth. Another example of unconditioned response would be jumping or screaming when someone scares us.