You use an electric can opener to open your cat's food every day. When you turn on the can opener and it starts to make noise, your cat comes running into the kitchen to eat his food. Today, you used the can opener to open a can of tuna to make yourself a sandwich for lunch, and the cat came running into the kitchen. Your cat has come to associate the previously neutral stimulus of the electric can opener with his cat food. The cat food is the:

Positive reinforcement

Explanation:

This is called classic conditioning first described by Pavlov. In his experiment a bell would ring a bell: stimuli then feed the dog and after a few incidences of this the dog would salivate in anticipation for food: reward. In this case the sound of the can opener would be the stumuli and the cat food is the reward which is the positive reinforcement since they behave in a desired way. i.e coming when called/hearing sound of can opener.