**Core Concept**
The instrument in question is a specialized tool used in orthopedic surgery for managing non-union sites, which are areas of bone that fail to heal properly. The instrument is designed to remove fibrous tissue, a type of scar tissue that can hinder the healing process.

**Why the Correct Answer is Right**
The instrument is specifically designed as a nibbler, which allows for the controlled removal of fibrous tissue from non-union sites. This is crucial in orthopedic surgery as fibrous tissue can prevent the bone from healing properly, leading to prolonged recovery times and potential complications. By removing this tissue, the instrument promotes a conducive environment for bone healing.

**Why Each Wrong Option is Incorrect**
**Option A:** Cutting ribs is a procedure typically performed in thoracic surgery, not spinal surgery, and requires a different set of instruments. The instrument in question is not designed for this purpose.

**Option B:** Smoothening of edges is a common procedure in amputation surgery, but it requires a different type of instrument. The instrument in question is specifically designed for removing fibrous tissue, not for smoothing edges.

**Option D:** Holding hard tissues is a function of various orthopedic instruments, such as clamps or retractors. The instrument in question is not designed for this purpose, but rather for removing fibrous tissue.

**Clinical Pearl / High-Yield Fact**
When managing non-union sites, it's essential to identify and remove fibrous tissue to promote bone healing. This can be achieved using specialized instruments like the one in question, which are designed to nibble away at the tissue without causing further damage.

**✓ Correct Answer: C. Used as nibbler and remove fibrous tissue from non-union site**