Answer:
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".