Joseph used classical conditioning to teach his sister to be afraid of squirrels. if he is successful, the neutral stimulus, the squirrel, must have been paired with a(n) such as

In order to carry out classical conditioning, a neutral stimulus must be paired with an unconditioned stimulus, such as a loud noise.
By combining the neutral stimulus with the unconditioned one, a person or animal may be made to associate the two stimuli subconsciously. This is also observed in children that are taught to fear an object or an animal, where a neutral stimulus such as a cat may be paired with an unconditioned stimulus, such as the pain of being scratched.

If you want the answer you need to post the figure above that it's talking about and you