
Answer:
The dog leash has become the conditioned stimulus
Explanation:
In classical/respondent conditioning, antecedent events become associated with one another.
A stimulus that does not typically produce a response is paired with one that does.
Learning is said to have taken place when the stimulus that did not produce a response begins to elicit a response.
In this scenario, the stimulus that does not elicit a response (neutral stimulus) is the dog's leash and after a while, it becomes the conditioned stimulus.
Rattling the chain is the stimulus that has been paired with walking the dog.