The correct option is C Is released when blood calcium levels fall
Parathyroid hormone is secreted by the parathyroid glands and is the most important regulator of calcium levels in the blood and within the bones. Parathyroid hormone is mainly controlled by the negative feedback of calcium levels in the blood to the parathyroid glands. Low calcium levels in the blood stimulate parathyroid hormone secretion, whereas high calcium levels in the blood prevent the release of parathyroid hormone.