**Core Concept**
Bone cysts, such as odontogenic keratocysts or radicular cysts, require enucleation to remove the cystic lining and relieve pressure on surrounding bone. Following enucleation, the bone cavity must be filled to maintain its structural integrity and promote healing.

**Why the Correct Answer is Right**
The substance filled in the bone cavity after enucleation is usually **Bone Graft**. This is typically a mixture of autogenous bone (harvested from the patient's own body) or allogenic bone (from another individual) that is placed into the cavity to promote bone healing and regeneration. The bone graft serves as a scaffold for new bone growth, allowing the bone to remodel and regain its original strength.

**Why Each Wrong Option is Incorrect**
**Option A:** **Bone Cement** is used in orthopedic surgery to stabilize and secure prosthetic joints, but it is not typically used to fill bone cavities after cyst enucleation.

**Option B:** **Hydroxyapatite** is a type of calcium-based biomaterial used in dental and orthopedic applications, but it is not the primary substance used to fill bone cavities after cyst enucleation.

**Option C:** **Marrow** is a component of bone tissue, but it is not typically used as a standalone substance to fill bone cavities after cyst enucleation.

**Clinical Pearl / High-Yield Fact**
When filling a bone cavity after cyst enucleation, it is essential to use a substance that promotes bone healing and regeneration. Autogenous bone grafts are often preferred due to their osteoconductive and osteoinductive properties, which facilitate new bone growth and integration with surrounding bone tissue.

**Correct Answer:** C. Bone Graft.