Which of the following is an example of an unconditioned stimulus in classical conditioning?

In classical conditioning, an unconditioned stimulus (US) is something that naturally and automatically triggers a response without any prior learning or conditioning needed. In the context of the options provided, a puff of air to the eye qualifies as an unconditioned stimulus because it provokes a reflexive blink response that occurs naturally. This does not require any previous conditioning or learning; the blink is an innate reaction to the puff of air.

The other options represent different stimuli but do not exhibit the same automatic response characteristics in the context of classical conditioning. The sound of a bell is often used as a conditioned stimulus, especially in the famous Pavlov experiment, where it became associated with food. A light being turned on can also serve as a neutral stimulus but does not evoke an immediate unlearned response. Similarly, while a pleasant taste of food can elicit a positive response, it is more associated with being a conditioned response when paired with a neutral stimulus rather than acting as an unconditioned stimulus itself. Thus, the puff of air serves as the clear example of an unconditioned stimulus due to its innate ability to produce a reflex action.