**Core Concept**
Osteosarcoma is a malignant bone tumor characterized by abnormal bone formation, which can be visualized on radiographs. The radiological features of osteosarcoma are crucial for its diagnosis and differentiation from other bone lesions.

**Why the Correct Answer is Right**
The correct answer is **B. Sun ray appearance**. This is due to the characteristic periosteal reaction seen in osteosarcoma, where new bone forms in a linear fashion, resembling sun rays. This occurs as a result of the rapid growth of the tumor, which leads to the lifting of the periosteum and subsequent new bone formation. The sun ray appearance is a classic radiological feature of osteosarcoma and is often seen on plain radiographs.

**Why Each Wrong Option is Incorrect**
* **Option A:** New bone formation is a characteristic feature of osteosarcoma, but it is not as specific as the sun ray appearance. New bone formation can also be seen in other bone lesions such as osteoblastoma and fibrous dysplasia.
* **Option C:** Codman's triangle is a radiological feature of osteosarcoma, but it is not as specific as the sun ray appearance. Codman's triangle is formed by the lifting of the periosteum and new bone formation, but it can also be seen in other bone lesions such as Ewing's sarcoma.
* **Option D:** Soap bubble appearance is characteristic of lytic bone lesions such as multiple myeloma and metastatic disease, but it is not typically seen in osteosarcoma.

**Clinical Pearl / High-Yield Fact**
When evaluating a bone lesion, it is essential to look for characteristic radiological features such as the sun ray appearance in osteosarcoma. This can help differentiate it from other bone lesions and guide further management.

**Correct Answer:** B. Sun ray appearance