**Core Concept**
The question is testing the student's knowledge of bone tumors, specifically the characteristics of osteosarcoma. Osteosarcoma is a type of malignant bone tumor that arises from osteoid-producing cells and is the most common primary malignant bone tumor in children and adolescents.

**Why the Correct Answer is Right**
The presence of a radiolucent area with Codman's triangle and Sunray appearance on X-ray is highly suggestive of osteosarcoma. Codman's triangle is a triangular radiolucent area that forms at the junction of the tumor and the normal bone, while Sunray appearance refers to the radiating, thin, wispy lines that represent periosteal new bone formation in response to the tumor. These radiographic features are classic for osteosarcoma, which arises from the medullary cavity and invades the surrounding bone, leading to periosteal reaction and new bone formation.

**Why Each Wrong Option is Incorrect**
* **Option A:** Osteochondroma is a benign bone tumor that typically presents as a bony projection covered by cartilage, and does not typically show Codman's triangle or Sunray appearance on X-ray.
* **Option B:** Ewing's sarcoma is another malignant bone tumor that typically presents with a moth-eaten or lytic appearance on X-ray, but does not typically show Codman's triangle or Sunray appearance.
* **Option C:** Fibrous dysplasia is a benign bone disorder that presents with a ground-glass appearance on X-ray, but does not typically show Codman's triangle or Sunray appearance.

**Clinical Pearl / High-Yield Fact**
In children and adolescents, a bone lesion with Codman's triangle and Sunray appearance on X-ray is highly suggestive of osteosarcoma. This diagnosis requires prompt evaluation and referral to an orthopedic oncologist for further management.

**Correct Answer: C. Osteosarcoma**