You want to condition a pet pig to come running for a food reward when you blow a whistle. In the process of this conditioning effort, the main idea is to A. pair a conditioned stimulus with an unconditioned stimulus. B. teach the pig to expect food when it's hungry. C. teach the pig to pay attention to the sound of a whistle. D. pair a neutral stimulus with an unconditioned stimulus.

The main idea of that conditioning effort mentioned in the question is to pair a neutral stimulus with an unconditioned stimulus, because the neutral stimulus would be when a whistle blows, because although the pig has a natural response towards the whistle sound (because it would orient its head towards the source of the sound), nevertheless, the pig has a neutral response with respect to the answer that we tried to condition (for example come running for a food reward when the whistle blows).

So, the main idea of that conditioning effort is to pair that Neutral Stimulus (the wanted behavior when the whistle blows) with an Unconditioned Stimulus (that it would be come running for a food reward, because come running for food is a natural, unlearned or unconditioned response that all animals do when someone will give them food), with the only purpose to generate as a result of the link between a Neutral stimulus and an Unconditioned stimulus, a Conditioned (Or learned) Response in the pig.

Then, according to the previous, the correct answer is option D. pair a neutral stimulus with an unconditioned stimulus.