**Core Concept**
Ewing's sarcoma is a malignant bone tumor that primarily affects children and young adults, characterized by its aggressive behavior and tendency to metastasize. It arises from the primitive mesenchymal cells, which are the precursors to bone and cartilage.

**Why the Correct Answer is Right**
The most common site of Ewing's sarcoma is the diaphysis of long bones, particularly the femur, tibia, and humerus. This is due to the high concentration of primitive mesenchymal cells in these areas, which are more susceptible to malignant transformation. The tumor typically presents as a painful, swollen mass with a characteristic "onion-skin" periosteal reaction on radiographs.

**Why Each Wrong Option is Incorrect**
**Option A:** Pelvis - While Ewing's sarcoma can occur in the pelvis, it is not the most common site.
**Option B:** Rib - Rib involvement is relatively rare in Ewing's sarcoma.
**Option C:** Spine - Although Ewing's sarcoma can affect the spine, it is not typically located in this area.

**Clinical Pearl / High-Yield Fact**
Ewing's sarcoma is a highly malignant tumor that often presents with systemic symptoms such as fever, weight loss, and fatigue, in addition to local pain and swelling.

**Correct Answer: C. Diaphysis of long bones, especially femur, tibia, and humerus.**