**Core Concept**
Codman's triangle is a radiological sign observed in osteosarcoma, a type of malignant bone tumor. It represents a characteristic triad of bone destruction, periosteal reaction, and new bone formation. The periosteum, a fibrous membrane covering the bone, reacts to the tumor by lifting off and forming a new bone layer, creating a triangular shape on radiographs.

**Why the Correct Answer is Right**
The correct answer is related to the aggressive behavior of osteosarcoma, which leads to the destruction of the underlying bone matrix and the periosteal reaction. The periosteum, in response to the tumor, lifts off and forms a new bone layer, visible as Codman's triangle on X-rays. This phenomenon is a result of the rapid growth and invasion of the tumor cells into the surrounding bone tissue.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Codman's triangle is not typically associated with other types of bone tumors, such as giant cell tumors or aneurysmal bone cysts, which have different radiological features.

**Option B:** This option is incorrect because Codman's triangle is not a characteristic feature of bone metastases, which often present as lytic or blastic lesions without a periosteal reaction.

**Option C:** This option is incorrect because Codman's triangle is not a feature of fibrous dysplasia, a benign bone disorder characterized by fibrous tissue replacement of normal bone.

**Clinical Pearl / High-Yield Fact**
Remember that Codman's triangle is a classic radiological sign of osteosarcoma, but it can also be seen in other aggressive bone tumors. Always consider the clinical context and other radiological features when interpreting X-rays.

**Correct Answer: D. Osteosarcoma**