**Core Concept**
In radiology, the appearance of the femur on an X-ray can provide clues about the underlying pathological process. The Codman triangle is a characteristic radiographic feature that is seen in certain types of bone lesions.

**Why the Correct Answer is Right**
The Codman triangle is a triangular zone of bone resorption that forms when a bone tumor or lesion erodes the periosteum, causing new bone to form in the subperiosteal space. This results in a characteristic radiographic appearance of a triangular shape with the base at the tumor margin and the apex at the site of original bone. The Codman triangle is classically seen in osteosarcoma, a type of primary bone cancer. The formation of the Codman triangle is due to the rapid growth of the tumor, which causes the periosteum to be lifted off the bone, leading to new bone formation in the subperiosteal space.

**Why Each Wrong Option is Incorrect**
**Option A:** Sunburst calcification is a radiographic feature seen in osteosarcoma, but it is not the same as the Codman triangle. Sunburst calcification refers to the appearance of calcification that radiates out from the center of the tumor, giving a characteristic "sunburst" appearance.

**Option B:** Onion peel effect is a radiographic feature seen in fibrous dysplasia, a condition characterized by the replacement of normal bone with fibrous tissue. This results in a characteristic "onion-skin" appearance of the bone on X-ray.

**Option D:** Bone marrow invasion is a feature that can be seen on imaging studies, such as MRI or CT scans, but it is not a specific radiographic feature that is seen on X-ray.

**Clinical Pearl / High-Yield Fact**
The Codman triangle is a classic radiographic feature of osteosarcoma, and its presence should raise suspicion for this diagnosis. It is essential to consider the clinical context and other imaging findings to make an accurate diagnosis.

**✓ Correct Answer: C. Codmann triangle**