**Core Concept**
Rhabdomyosarcoma is a type of soft tissue sarcoma that arises from the skeletal muscle cells, and in children, it is the most common primary orbital tumour. The orbital location is a result of the embryological development of the eye and surrounding tissues.

**Why the Correct Answer is Right**
Rhabdomyosarcoma, specifically the embryonal subtype, is the most common primary orbital tumour in children, accounting for approximately 5-8% of all childhood sarcomas. The tumour arises from the extraocular muscles or the orbital soft tissues, often presenting with proptosis, swelling, and limited extraocular movement. The embryonal subtype is particularly aggressive and has a high propensity for local invasion and metastasis.

**Why Each Wrong Option is Incorrect**
**Option A:** Optic nerve sheath meningioma is a common primary orbital tumour in adults, particularly in middle-aged women, but is rare in children.
**Option B:** Retinoblastoma is a malignant tumour of the retina, primarily affecting young children, but it is not a primary orbital tumour in the classical sense.
**Option D:** Glioma of the optic nerve is a rare, benign tumour of the optic nerve sheath, more commonly seen in adults and is not the most common primary orbital tumour in children.

**Clinical Pearl / High-Yield Fact**
Rhabdomyosarcoma is a classic example of a "masquerader" tumour, often mimicking other conditions such as thyroid ophthalmopathy or orbital cellulitis, highlighting the importance of thorough imaging and histopathological examination in diagnosing orbital tumours.

✓ Correct Answer: C. Rhabdomyosarcoma