**Core Concept**
Codman's triangle is a radiological sign that represents a characteristic appearance of bone formation and destruction at the site of a tumor, particularly osteosarcoma. It is formed by the new bone formation at the edge of the tumor, the cortex of the bone, and the periosteum. This sign is essential in diagnosing bone tumors, especially in the early stages.

**Why the Correct Answer is Right**
The formation of Codman's triangle occurs due to the reactive new bone formation at the edge of the tumor, which is a response to the tumor's invasion into the surrounding bone. This new bone formation is mediated by osteoblasts, which are responsible for bone deposition. The periosteum, a layer of connective tissue surrounding the bone, also plays a crucial role in the formation of Codman's triangle. As the tumor grows, it lifts the periosteum, leading to the formation of a triangle-shaped radiolucent area on the X-ray.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Codman's triangle is not typically associated with benign bone tumors, such as osteochondroma. While osteochondroma can cause bone formation, it does not typically exhibit the characteristic Codman's triangle sign.
**Option B:** This option is incorrect because Codman's triangle is not a feature of bone fractures. While fractures can cause bone formation and callus formation, they do not typically exhibit the characteristic Codman's triangle sign.
**Option C:** This option is incorrect because Codman's triangle is not a feature of bone cysts, such as unicameral bone cysts. Bone cysts can cause bone destruction, but they do not typically exhibit the characteristic Codman's triangle sign.

**Clinical Pearl / High-Yield Fact**
Codman's triangle is a classic radiological sign of osteosarcoma, a type of bone cancer that is often seen in young adults. When interpreting X-rays of the bone, it is essential to look for this sign, as it can be a crucial diagnostic clue.

**Correct Answer:** D.