The hormone 'calcitonin' is secreted by:

Explanation

Option A is correct. The hormone calcitonin is secreted by the thyroid gland. The thyroid gland is a small, butterfly-shaped gland located in the neck. It is responsible for producing hormones that regulate metabolism, growth, and development, and body temperature. Calcitonin is a hormone that helps to regulate calcium levels in the blood. It works by inhibiting the activity of osteoclasts, which are cells that break down bone. This helps to prevent calcium from being released into the blood from bone. Calcitonin also helps to increase the excretion of calcium in the urine. This also helps to lower calcium levels in the blood. The other options are not involved in the secretion of calcitonin: Parathyroid gland: The parathyroid gland produces parathyroid hormone, which is a hormone that helps to raise calcium levels in the blood. Pancreas: The pancreas produces insulin and glucagon, which are hormones that regulate blood sugar levels. Adrenal gland: The adrenal gland produces hormones such as cortisol, adrenaline, and aldosterone, which are involved in a variety of functions, including stress response, metabolism, and blood pressure regulation.