Answer:
unconditioned stimulus.
Step-by-step explanation:
The unconditioned stimulus is defined as a process that naturally, automatically, and unconditionally triggers a response.
Example: When you smell the food of your choice or the one you like to eat, you may feel hungry immediately. In the given example, the unconditioned stimulus is the smell of the food.
The smell of the food causes salivation is an example of a naturally occurring stimulus.