Final answer:
Ghrelin is the hormone that stimulates specific brain structures to induce hunger, acting on the hypothalamus region of the brain, while insulin, leptin, and peptide YY are related to satiety.
Step-by-step explanation:
For hunger, the biochemical agent that best illustrates the principle of "biochemical agents stimulate specific brain structures" is D. Ghrelin. Ghrelin is a hormone that is produced and released mainly by the stomach with small amounts also released by the small intestine, pancreas, and brain. Ghrelin levels increase before meals and decrease after meals, and it functions as a signal to the brain to stimulate appetite. This hormone acts on the hypothalamus region of the brain to induce the feeling of hunger. On the other hand, Insulin and Leptin both contribute to the feeling of satiety, with insulin being released by the pancreas when blood glucose levels rise, and leptin being released by fat cells as food passes through the gastrointestinal tract. Peptide YY is another hormone involved in reducing appetite but is released by the digestive tract in response to food intake.