**Core Concept**
Osteoclasts are bone-resorbing cells that respond to hormonal signals regulating calcium homeostasis. The receptor for calcitonin on osteoclasts is critical in suppressing bone resorption, thereby reducing blood calcium levels.

**Why the Correct Answer is Right**
Calcitonin binds to specific receptors on osteoclasts, triggering intracellular signaling pathways that inhibit osteoclast activity and reduce bone resorption. This action decreases calcium release from bone into the bloodstream. The calcitonin receptor is a G-protein-coupled receptor (GPCR) that activates adenylate cyclase inhibition and reduces cyclic AMP (cAMP), leading to osteoclast suppression. This mechanism is vital in treating hypercalcemia.

**Why Each Wrong Option is Incorrect**
Option A: Parathyroid hormone (PTH) acts on osteoclasts indirectly via PTH receptors, but it stimulates bone resorption, not through a "specific receptor" in the same way as calcitonin. PTH increases bone breakdown, not suppresses it.
Option C: Thyroxin (T4) influences bone metabolism indirectly by affecting growth and calcium metabolism but does not bind to osteoclast receptors. Its effect is mediated through central nervous system and metabolic pathways, not direct osteoclast signaling.
Option D: Vitamin D3 (calcitriol) enhances calcium absorption in the gut and promotes bone mineralization, but it acts on osteoblasts and intestinal cells, not directly on osteoclast receptors. It indirectly modulates bone turnover but does not have a specific receptor on osteoclasts.

**Clinical Pearl / High-Yield Fact**
Calcitonin is the only hormone with a direct receptor on osteoclasts that *suppresses* bone resorption—this makes it key in treating hypercalcemia and osteoporosis. PTH stimulates resorption, so understanding this contrast is essential for managing bone metabolism disorders.

✓ Correct Answer: B. Calcitonin