Answer:
O c. a conditioned reinforcer
Explanation:
Based on the information given the stimulation of the rat's brain is what is called A CONDITIONED REINFORCER reason been that CONDITIONED REINFORCER is a stimulus REINFORCER which enables signal to be send out whenever the lever is pressed as well as when the red light is turned on which means that the lever work hand in hand with the red light which in turn lead to the stimulation of the rat brain thereby reinforcing the pleasure in the rat brain reward centre without stopping the pleasure and reward the rat's received because the rat's have electrode implanted in their brain.