Final answer:
The conditioned stimulus in Harmony's situation is the sound of the electric can opener, as the cat has learned to associate this previously neutral sound with receiving food, leading to salivation.
Step-by-step explanation:
In the scenario where Harmony's cat salivates upon hearing the sound of an electric can opener, the conditioned stimulus is D. the sound of the electric can opener. This process is an example of classical conditioning, first studied by Ivan Pavlov, where a neutral stimulus (the sound of the can opener) becomes associated with an unconditioned stimulus (the food), which leads to an unconditioned response (salivation). Over time, after repeated pairings, the neutral stimulus alone can elicit the response, now called the conditioned response.
Classical conditioning involves the pairing of a previously neutral stimulus with an unconditioned stimulus to elicit a natural response. When the neutral stimulus is able to elicit that response on its own, it is then known as the conditioned stimulus. In this case, Harmony's cat has learned to associate the sound of the electric can opener with being fed, which is why it begins to salivate upon hearing the noise.