Final answer:
Ghrelin is known as an appetite stimulator that causes increased food intake, while leptin is a satiety hormone secreted by adipose tissues, which inhibits appetite when fat reserves are high. Leptin resistance can lead to overeating and obesity.
Step-by-step explanation:
The powerful appetite stimulator causing people to hoard food is Neuropeptide Y (b), which is not one of the options provided in the question. However, among the listed options, Ghrelin (d) is a hormone known for stimulating appetite, increasing food intake, and promoting fat storage. In contrast, Leptin (a), which is produced by adipose tissues, signals satiety and inhibits appetite when fat reserves are high, thus contributing to the regulation of body weight. Leptin resistance is a condition often seen in obesity, where the body becomes less sensitive to leptin and the person does not feel the satiety despite having ample fat reserves.
Furthermore, low blood sugar levels stimulate certain neurotransmitters that indicate fullness and reduce hunger, and these are often related to hormones like insulin, not leptin. The hypothalamus region of the brain controls satiety signals, influenced by hormone levels and nutritional content of foods, particularly fatty acids.