**Core Concept**
Codman's triangle is a radiological sign observed in cases of bone fractures, particularly in the context of osteosarcoma. It is characterized by a triangular area of new bone formation at the site of a fracture, resulting from the periosteum's attempt to repair the damaged area.

**Why the Correct Answer is Right**
Codman's triangle is formed when a tumor, such as osteosarcoma, invades the bone and causes a fracture. The periosteum, a layer of connective tissue covering the bone, responds to the injury by producing new bone tissue. However, since the tumor is aggressive and invasive, it prevents the periosteum from forming a complete new bone layer, resulting in a characteristic triangular shape. This sign is often visible on X-rays and is a key diagnostic feature of certain types of bone tumors.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Codman's triangle is not typically associated with benign bone lesions, such as osteoid osteoma.
**Option B:** This option is incorrect because Codman's triangle is not a feature of bone metastases, which typically present with lytic or blastic lesions without a clear periosteal reaction.
**Option C:** This option is incorrect because Codman's triangle is not a feature of non-union fractures, which typically present with a lack of bony healing or a fracture gap, rather than a new bone formation.

**Clinical Pearl / High-Yield Fact**
When interpreting X-rays of the bone, it's essential to recognize Codman's triangle as a sign of aggressive bone lesions, particularly osteosarcoma. This sign can aid in the early detection and diagnosis of these tumors.

**Correct Answer: D. Osteosarcoma.**