The correct answer is: A. Calcitonin
Calcitonin is a thyroid hormone that helps to regulate the levels of calcium in the blood. It does this by inhibiting the release of calcium from the bones and by increasing the absorption of calcium from the intestines. Calcitonin is also involved in the metabolism of bone and in the maintenance of bone health.
Thyroxine is another thyroid hormone, but it does not have a direct effect on calcium levels. Thyroxine is involved in the regulation of metabolism and growth. Calmodulin is a protein that is involved in the regulation of many cellular processes, including the regulation of calcium levels. However, calmodulin is not a thyroid hormone.
Therefore, the only option that is a thyroid hormone that controls the balance of calcium in the body is calcitonin.