What method did Skinner use to encourage the rat to press the lever?

Skinner utilized positive reinforcement through food to encourage the rat to press the lever. In his experiments, he placed a rat in a specially designed box, which included a lever that could be pressed. Whenever the rat pressed the lever, a food pellet was dispensed. This immediate reward for the desired behavior of pressing the lever effectively increased the frequency of that behavior, demonstrating the principles of operant conditioning.

This method aligns with Skinner's theory that behaviors followed by favorable consequences are more likely to be repeated. Positive reinforcement strengthens behavioral responses by providing a rewarding stimulus, in this case, food, which is a primary reinforcer for the rat. The rat learns to associate lever pressing with receiving food, thereby increasing the likelihood of that behavior occurring again.

The other options reflect different concepts in behavioral psychology. Physical punishment and negative reinforcement generally involve removing an unpleasant stimulus or administering an aversive consequence, which does not apply in Skinner's method here. Rewarding with social interactions also does not fit, as the reinforcement used in this scenario was tangible (food) rather than social.