Which of the following conditions will stimulate parathyroid gland to release parathyroid hormone?

Parathyroid hormone is a hypercalcemic hormone. It increases blood Ca2+ levels by stimulating osteoclasts. These cells cause bone resorption. The secretion of parathyroid hormone is regulated by blood calcium level. A fall in blood Ca2+levels stimulate the release of parathyroid hormone leading to release  Ca2+into the blood in order to maintain calcium homeostasis. A rise in Ca2+ in blood stimulates the release of thyrocalcitonin which decreases the number and activity of osteoclasts and thereby reducing the Ca2+ levels in the blood.