In a classical conditioning experiment, a puff of air was blown into Rudy's eye and he reflexively blinked. The experimenter then began flashing a green light just before presenting the puff of air. After many pairings of the green light and the puff of air, Rudy began to blink as soon as the green light appeared, whether or not the air puff followed. In this experiment, the green light is the:

Conditioned stimulus

Explanation:

Conditioned stimulus: In psychology, the term has been used in the classical conditioning experiment which was developed by one of the famous and most influential psychologists named Ivan Pavlov while experimenting with dogs, and the term is often referred to as CS.

In a classical conditioning experiment, the conditioned stimulus is defined as a formerly neutral stimulus that is being paired or associated with an unconditioned stimulus to trigger or produce a conditioned response.

In the question above, the green light is the conditioned stimulus.