The parathyroid glands are glands of the human endocrine system that are located behind the thyroid glands in the neck. The main function of the parathyroid glands is to control the amount of calcium in the body (mainly bones and blood). According to the above, the parathyroid glands produce essential hormones for the proper function of the bone system because it controls the calcium that is used to strengthen the bones. These at the same time serve as the storage system to ensure that the body always has a vast source of calcium. So the correct answer is the D. Skeletal system.
