At home, you rattle the chain on your dog’s leash every time you prepare to take him for a walk. After several episodes like this, you find that your dog comes running to the front door even when you pick up the leash to put it back in the closet. In this example, what is the conditioned stimulus?

the sound of the leash                        

Explanation:

Conditioned stimulus: The term "conditioned stimulus" is determined as one of the different parts in the "classical conditioning theory" which was introduced by Ivan Pavlov while demonstrating his experiment on the laboratory dogs.

Conditioned stimulus is referred to as a stimulus that was neutral formerly and once it becomes connected with an unconditioned stimulus then it gives rise to a conditioned response.

In the question above, the conditioned stimulus is "the sound of the leash".