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.