Answer: Sound of the drill
Step-by-step explanation:
The Conditioned stimulus is the occurrence that has been associated with the Unconditioned stimulus to bring out a conditioned reaction.
The Unconditioned stimulus in this scenario is the pain that was experienced when the dentist worked on the teeth. This has now been associated with the sound of a drill such that whenever Jill hears this sound, he associates it with Pain and becomes anxious.