If you are not getting adequate calcium in your diet, calcitonin will be secreted, which stimulates osteoclasts to resorb bone and thereby increases calcium levels in the blood.

The statement above is false. The blood levels of calcium is regulated by parathyroid hormone.When one is not getting adequate calcium in one's diet and the blood level of calcium is low, the parathyroid hormone will be released. The parathyroid hormone can increase the blood calcium in several ways:
1. by stimulating osteoclasts which break down bone and release calcium into the blood stream.
2. by increasing the amount of calcium re-absorbed by the kidney before it can be excreted in urine.
3. by triggering the formation of calcitriol which increases the absorption of dietary calcium at the  intestines.

Ithink the answer is d.

C- a hypothesis or d- an observation