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

The release of Parathyroid Hormone ($\text{PTH}$) is regulated by the circulating levels of calcium ions. A fall in blood calcium (${\text{Ca}}^{2+}$) levels stimulates the parathyroid glands to secrete $\text{PTH}$, which then acts to increase blood calcium by mobilizing it from bones and increasing reabsorption in kidneys.