A college student throws a ball at his roommate's head every time the phone rings. After a while, the roommate begins to flinch involuntarily at the sound of the phone. In this example, getting hit by the ball is the:

Unconditional Stimulus

Explanation:

In the first classical conditioning experiments by Ivan Pavlov that involved the sounding of buzzer whenever it was time to feed the dogs to induce salvation first in the dogs. The dogs were later conditioned to salivate at the sound of the buzzer even when there was no food.

Just like in this case, the college roommate uses the thrown ball to condition his roommate to clinch at the sound of the phone even when no ball would be thrown.

In this case,

- The ring of the phone is the conditioned stimuli. The stimuli that the subject is being trained to react to.

- The flinching of the roommate is the conditioned response to the conditioned stimuli.

- The getting hit by the ball is the unconditional stimuli. It is a perturbation whose natural reaction to it, is used to elicit the conditional response to a conditional stimuli.

It is the stimulus that naturally triggers a response.

- The reaction after getting hit with the ball (unconditional stimuli) is the unconditional response. It is the natural response to pain worldwide.