In learning, Pavlov’s conditioning theory, he experimented on:

Classical conditioning (also called Pavlovian or respondent conditioning) is a form of learning in which a neutral stimulus, when repeatedly paired with an unconditioned stimulus that naturally triggers a reflex response, comes to trigger that same response by itself, now called a conditioned response to a conditioned stimulus.

Ivan Pavlov demonstrated this principle experimentally: he paired the sound of a bell, a neutral stimulus, with the presentation of food, an unconditioned stimulus, to a dog, which naturally salivated at the food, an unconditioned response. After several pairings, the bell alone made the dog salivate, so the sound had become a conditioned stimulus producing a conditioned response. This conditioned-reflex research was carried out on the dog.

Each of the other animals offered here is tied to a different, unrelated line of learning research, not to Pavlov's conditioned-reflex work:

• Cat: associated with Edward Thorndike's puzzle-box experiments, which studied trial-and-error learning and the Law of Effect, not conditioned reflexes.

• Monkey: associated with Harry Harlow's attachment experiments on rhesus monkeys, a separate line of research on social bonding, outside the conditioned stimulus-response paradigm Pavlov established.

• Rat: associated with B. F. Skinner's operant-conditioning research, the Skinner box, which studies how voluntary behaviour is shaped by reinforcement, a different paradigm from Pavlov's reflex-pairing model.

Since Pavlov's own experiments specifically produced a conditioned salivation reflex in dogs, the dog is the correct subject of his theory.