The correct answer is d. A conditioned response.
Ivan Pavlov was a Russian scientist interested in studying how digestion works in mammals. According to Pavlov, the dogs were demonstrating a conditioned response. He explained it like this: there's a neutral stimulus (the bell), which by itself will not produce a response, like salivation. There's also a non-neutral or unconditioned stimulus (the food), which will produce an unconditioned response (salivation).
