Final answer:
In operant conditioning, the green light is a conditioned stimulus (Sd) that signals the availability of food. The rat learns to associate the green light with receiving food, which reinforces the behavior of pressing the lever. So, the correct answer is option c.
Step-by-step explanation:
In operant conditioning, the conditioned behavior is gradually modified by its consequences as the animal responds to the stimulus. In the case of the hungry rat, the green light acts as a conditioned stimulus (Sd). When the rat presses the lever and the green light is on, it receives food as a reward, which reinforces the behavior of pressing the lever. So, the correct answer is option c.