Final answer:
In the context of Harry stroking Tabby, the cat's purr serves as a positive reinforcement in operant conditioning, leading Harry to repeat the behavior.
Step-by-step explanation:
In the scenario described where Harry strokes Tabby and the cat begins to purr, and as a result, he frequently strokes Tabby softly, this is an example of operant conditioning. Operant conditioning is a type of learned behavior where an animal's behavior is shaped by the consequences of that behavior. Specifically, it involves a method of learning that employs rewards or punishments for behavior. In the case of Harry and the cat, Tabry's purring serves as a positive reinforcement that encourages Harry to repeat the action of stroking. This reinforcement strengthens the association between the action (stroking) and the positive outcome (the cat's purring), which increases the likelihood that the behavior will occur again.