Concept:The parathyroid glands produce parathyroid hormone, which regulates blood calcium levels.Explanation:The parathyroid glands secrete parathyroid hormone (PTH), which increases blood calcium by stimulating bone resorption, kidney calcium reabsorption, and vitamin D activation.The thyroid gland produces calcitonin, which lowers blood calcium, but the primary regulators of calcium balance include the parathyroid glands.Answer:D. parathyroid