The correct answer is D. Insulin.
Insulin is a hormone that is produced by the pancreas. It helps the body to use glucose for energy. When you eat, your blood sugar levels rise. Insulin helps to lower your blood sugar levels by allowing your cells to take in glucose from the bloodstream. Insulin also helps to store glucose in your liver and muscles for later use.
Glucagon is a hormone that is also produced by the pancreas. It helps to raise your blood sugar levels when they are too low. Glucagon does this by causing your liver to release glucose into the bloodstream.
Thyroxine is a hormone that is produced by the thyroid gland. It helps to regulate your metabolism. Thyroxine helps your body to use energy and to produce heat.
Oxytocin is a hormone that is produced by the hypothalamus and released by the pituitary gland. It is involved in childbirth, breastfeeding, and social bonding.
In conclusion, insulin is the hormone that regulates the amount of glucose in the blood. Glucagon, thyroxine, and oxytocin are not involved in blood sugar regulation.