Rachel is training her dog, duke, to bark when there is a knock on the door. she already knows that the family cat will make duke bark, so she decides to knock on the door and then open the door to reveal the cat in an effort to condition duke to bark. what is serving as the unconditioned stimulus?

I think that Rachel is using the cat to condition the dog for appropriate times to bark. Therefore the knock at the door is serving as the unconditioned stimulus in the above described situation. She is trying to teach the dog to bark when there is a knock at the door by using the cat to induce barking.