**Core Concept**
Osteoclasts are large multinucleated cells responsible for bone resorption, playing a crucial role in bone remodeling. The regulation of osteoclast activity is essential to maintain bone health and prevent diseases such as osteoporosis.

**Why the Correct Answer is Right**
Osteoclasts are inhibited and modified by the RANKL (Receptor Activator of NF-κB Ligand) / OPG (Osteoprotegerin) system. RANKL, produced by osteoblasts, binds to its receptor RANK on osteoclasts, promoting their differentiation and activation. OPG, a decoy receptor for RANKL, inhibits osteoclast formation and activation by binding to RANKL, thereby regulating bone resorption. This delicate balance between RANKL and OPG is crucial for maintaining bone homeostasis.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect as it does not accurately describe the regulation of osteoclasts. While osteoblasts do interact with osteoclasts, the RANKL / OPG system is the primary mechanism of osteoclast regulation.
* **Option B:** This option is incorrect as it is a hormone involved in calcium and phosphate metabolism, but not directly involved in the regulation of osteoclasts.
* **Option C:** This option is incorrect as it is a cytokine involved in immune responses, but not directly involved in the regulation of osteoclasts.

**Clinical Pearl / High-Yield Fact**
The RANKL / OPG system is a key target for the development of osteoporosis treatments, such as denosumab, a monoclonal antibody that inhibits RANKL.

**Correct Answer:** C. The RANKL / OPG system.