Final answer:
The parafollicular cells of the thyroid gland secrete 3) calcitonin, which helps to lower blood calcium levels and promote bone growth.
Step-by-step explanation:
The hormone secreted by the parafollicular cells of the thyroid gland is calcitonin. These cells release calcitonin in response to high concentrations of calcium in the blood. The primary function of calcitonin is to lower blood Ca2+ levels and promote bone growth. It does this by stimulating the movement of calcium into bone and inhibiting osteoclast activity, which both help to decrease blood calcium levels.