**Core Concept**
The epiphysis is a secondary ossification center in long bones, which fuses with the metaphysis during growth. Tumors originating from the epiphysis are typically benign and cartilaginous in nature.

**Why the Correct Answer is Right**
Osteochondroma, also known as an exostosis, is a type of benign bone tumor that arises from the growth plate (epiphyseal plate) of long bones. It is characterized by the formation of a cartilaginous cap on the surface of the bone, which eventually ossifies. Osteochondroma is the most common primary bone tumor in children and adolescents, often presenting as a painless, asymptomatic mass on the metaphyseal end of the bone. The cartilaginous cap of osteochondroma is composed of undifferentiated cartilage cells, which eventually undergo endochondral ossification, forming a bony projection.

**Why Each Wrong Option is Incorrect**
**Option A:** Osteoclastoma (Giant Cell Tumor) typically occurs in the epiphyseal region but is more common in adults and is usually found at the ends of long bones, particularly the distal femur and proximal tibia. It is a malignant tumor in the majority of cases.
**Option B:** Chondroblastoma is a rare, benign tumor that primarily affects the epiphysis of long bones, but it is more commonly found in the metaphyseal region. It is characterized by a mixture of chondroid and fibrous tissue.
**Option D:** Ewing's sarcoma is a malignant bone tumor that typically affects the diaphysis or metaphysis of long bones, particularly in children and adolescents. It is characterized by a small, round, blue cell morphology.
**Option E:** Chondrosarcoma is a malignant tumor that arises from cartilage-producing cells, typically occurring in the metaphyseal region of long bones. It is more common in adults and is often found in the pelvis, femur, or humerus.

**Clinical Pearl / High-Yield Fact**
Osteochondroma is the most common primary bone tumor in children and adolescents, often presenting as a painless, asymptomatic mass on the metaphyseal end of the bone. It is essential to differentiate osteochondroma from other bone tumors, as it can mimic the clinical presentation of more aggressive tumors.

**Correct Answer: C. Osteochondroma**