Concept:Pavlov's classical conditioning experiment used a dog to study how a neutral stimulus can trigger a reflex through association.Explanation:Pavlov placed meat powder in a dog's mouth and measured saliva. He then rang a bell just before giving food. After repeated pairings, the dog salivated at the sound of the bell alone. This showed learned association between the bell (conditioned stimulus) and salivation (conditioned response). The dog was the subject of this experiment.Answer:D. Pavlov