**Core Concept**
Ewing's sarcoma is a malignant bone tumor characterized by its aggressive nature and specific histopathological features. It arises from the primitive neuroectodermal cells of the bone marrow. The tumor's location is critical in diagnosis and treatment planning.

**Why the Correct Answer is Right**
Ewing's sarcoma commonly arises in the diaphysis of long bones, particularly in the femur, tibia, and humerus. The tumor's predilection for the diaphyseal region is thought to be due to the presence of a unique population of stem cells in this area. The diaphysis is rich in blood supply, which may facilitate the rapid growth of the tumor. Additionally, the diaphyseal region is a site of active bone remodeling, which may contribute to the tumor's development.

**Why Each Wrong Option is Incorrect**
**Option A:** While Ewing's sarcoma can occur in the pelvis, it is not the most characteristic location of the tumor.
**Option B:** The metaphysis is a common location for other types of bone tumors, such as osteosarcoma and chondrosarcoma, but it is not typical for Ewing's sarcoma.
**Option C:** Ewing's sarcoma can occur in the spine, but this is less common than its occurrence in long bones.

**Clinical Pearl / High-Yield Fact**
The characteristic radiographic appearance of Ewing's sarcoma is a "moth-eaten" or "onion-skin" periosteal reaction, with a sclerotic border and a soft tissue mass.

**Correct Answer: D. Diaphysis**