The correct option is Melatonin.
Explanation
The pineal gland is a small endocrine gland located on the dorsal side of the forebrain. It plays a central role in the regulation of the body's biological rhythms.
- Melatonin Melatonin: Correct. The pineal gland secretes a hormone called melatonin. Melatonin plays a very important role in the regulation of a 24-hour (diurnal) rhythm of the body. For example, it helps in maintaining the normal rhythms of the sleep-wake cycle and body temperature. In addition, melatonin also influences metabolism, pigmentation, the menstrual cycle, and defense capability.
- Vasopressin Vasopressin: Incorrect. Vasopressin, also known as Antidiuretic Hormone (ADH), is synthesized by the hypothalamus and released by the posterior pituitary gland. It acts mainly at the kidney and stimulates resorption of water and electrolytes by the distal tubules, thereby reducing the loss of water through urine (diuresis).
- Gastrin Gastrin: Incorrect. Gastrin is a peptide hormone secreted by the G-cells of the stomach lining. It acts on the gastric glands and stimulates the secretion of hydrochloric acid and pepsinogen.
- Insulin Insulin: Incorrect. Insulin is a peptide hormone secreted by the $\beta$-cells of the Islets of Langerhans in the pancreas. It plays a major role in the regulation of glucose homeostasis by facilitating the uptake of glucose by cells, thereby lowering blood glucose levels.
Key Takeaway: Melatonin is the primary hormone responsible for regulating the circadian rhythm (sleep-wake cycle) and is secreted by the pineal gland.