The correct option is A Increase in serum calcium
The parathyroid glands secrete a peptide hormone called parathyroid hormone or parathormone (PTH). The release of parathromone is regulated by the circulating levels of calcium (Ca2+) ions in the blood. PTH plays an important role in calcium balance of the body. It maintains the calcium balance between the blood and other tissues. When the calcium level in the blood decreases, parathyroid hormone is released in order to increase the calcium level in the blood. PTH acts on bones and other skeletal structure and mobilizes the calcium (Ca2+) from the bones into the blood. This is called bone resorption/dissolution/demineralization. The parathromone also increases the reabsorption of calcium by the renal tubules and absorption of calcium from the digested food. Hence, parathormone induces increase in serum calcium.