You are conducting an informal study in which, for a month, you play a particular song for your best friend right before you serve dinner to her. eventually, you play the song, and then wait to see how your friend responds. she begins to salivate and says she has hunger pains. in this scenario, what is the unconditioned stimulus

In this case the unconditioned stimulus (UCS) is the food (dinner). 

An unconditioned stimulus (UCS) refers to a stimulus (such as food) that automatically and naturally produces a response (hunger in this case) without any learning. Another example of an UCS is the sound of metal plate suddenly falling on the floor. Such a stimulus naturally and automatically creates a loud noise which makes us response in a startled manner.