**Core Concept**
The question is testing knowledge of primary bone tumors, specifically the most common malignant variety. Primary bone tumors originate from bone tissue itself, whereas secondary bone tumors metastasize from other organs. The correct answer is a type of primary bone tumor characterized by uncontrolled cell growth.

**Why the Correct Answer is Right**
The correct answer is **Osteosarcoma**. Osteosarcoma is a malignant tumor that produces immature bone matrix. It arises from the osteoid tissue in the metaphysis of long bones, particularly in the distal femur, proximal tibia, or proximal humerus. The high cell turnover rate in the metaphysis of long bones makes it a common site for osteosarcoma development. The tumor cells exhibit a characteristic "sunburst" appearance under the microscope due to their rapid division and bone matrix production.

**Why Each Wrong Option is Incorrect**
* **Option A:** Osteochondroma is a type of benign bone tumor, not malignant.
* **Option B:** Chondrosarcoma is a malignant tumor, but it is less common than osteosarcoma.
* **Option D:** Ewing's Sarcoma is a malignant bone tumor, but it is more common in children and adolescents.

**Clinical Pearl / High-Yield Fact**
Osteosarcoma often presents with pain, swelling, and limited mobility in the affected limb. A classic radiographic feature is the "Codman triangle," which is a triangular area of bone resorption and new bone formation.

**Correct Answer: C. Osteosarcoma.**