**Core Concept**
A tumour arising from the epiphysis is a rare type of bone tumour that originates from the growth plate, a region of cartilage growth in the bone. The epiphysis is the end part of the long bone that is initially separated from the main part of the bone by a growth plate.

**Why the Correct Answer is Right**
The correct answer is a type of tumour known as a chondroblastoma. Chondroblastomas are benign (non-cancerous) tumours that typically occur in the epiphysis of long bones, such as the femur, humerus, or tibia. They are composed of immature cartilage cells and are often associated with the growth plate. The exact mechanism of chondroblastoma formation is not fully understood, but it is thought to involve abnormalities in the growth plate.

**Why Each Wrong Option is Incorrect**
**Option A:** Osteosarcoma - This is a type of malignant bone tumour that typically arises from the metaphysis (the part of the bone adjacent to the growth plate), not the epiphysis.

**Option B:** Giant cell tumour - This is a type of bone tumour that typically occurs in the epiphysis, but it is usually benign and more aggressive than chondroblastoma.

**Option C:** Ewing's sarcoma - This is a type of malignant bone tumour that typically occurs in the diaphysis (the shaft of the bone) or metaphysis, not the epiphysis.

**Clinical Pearl / High-Yield Fact**
Chondroblastomas often present in young adults with pain and swelling in the affected limb, and are typically diagnosed using imaging studies such as X-rays or MRI scans.

**Correct Answer: B. Giant cell tumour.**