Final answer:
Osteoclast activity is greater when calcitonin is absent, as calcitonin inhibits osteoclasts and promotes bone growth by stimulating osteoblasts.
Step-by-step explanation:
The effect of calcitonin on osteoclast activity is to inhibit it. Therefore, osteoclast activity is greater when calcitonin is absent. Calcitonin, which is produced by the thyroid gland, decreases blood calcium levels by inhibiting osteoclasts, stimulating osteoblasts, and encouraging calcium excretion by the kidneys. This hormone plays a significant role especially in children, during pregnancy, and during prolonged starvation.