The gland stimulates the release of calcium and phosphate ions into the blood when calcium levels become too low.

Parathyroid glands released the calcium and phosphate ions from bones into the blood when the concentration of calcium is low in the blood.

Explanation:

Parathyroid glands increases the release of osteoclasts which is a special type of cells that are responsible for the absorption of calcium that is stored in the bone and releases this calcium into the blood. When the concentration of calcium is too high, the bone again absorbed the calcium from the blood.

a

explanation:

b is the best answer for this question.