The correct answer is option C, that is, calcitonin stimulates the bones to absorb calcium when calcium levels are too low, while parathyroid hormone stimulates the bones to excrete calcium when calcium levels are too high.
Calcium homeostasis or calcium metabolism refers to the process by which the body sustains sufficient levels of calcium. Bone functions as an essential accumulation point for calcium, as it comprises 99 percent of the entire body calcium. The discharge of the calcium from bone is mediated by parathyroid hormone, while the incorporation of calcium in the bone is stimulated by calcitonin.
When the levels of calcium are low in bones, then calcitonin stimulates the bones to absorb calcium, on the other hand, when the levels of calcium are high in the bones, then parathyroid hormone stimulates the bones to remove excess calcium into the blood.