**Core Concept**
Retinoblastoma is a malignant tumor of the retina, and its treatment depends on the stage of the disease. Stage III is characterized by the presence of tumor in one eye with some extent of vitreous seeding.

**Why the Correct Answer is Right**
The treatment for stage III retinoblastoma typically involves enucleation, which is the surgical removal of the affected eye. This approach is chosen to prevent the spread of the tumor to other parts of the body. Enucleation is often performed in conjunction with cryotherapy or brachytherapy to ensure that all cancerous cells are eliminated. The goal of treatment is to preserve life while maintaining the best possible vision in the other eye.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as external beam radiation is typically reserved for more advanced stages of retinoblastoma, or in cases where enucleation is not feasible. It is not the primary treatment for stage III retinoblastoma.
**Option B:** This option is incorrect as systemic chemotherapy is often used in conjunction with other treatments, such as enucleation or radiation, but it is not the sole treatment for stage III retinoblastoma.
**Option C:** This option is incorrect as laser therapy is typically used for smaller tumors or for treating retinal detachment, but it is not effective for stage III retinoblastoma.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the treatment of retinoblastoma is highly dependent on the stage and extent of the disease. Early detection and prompt treatment can significantly improve outcomes for patients with this condition.

**Correct Answer:** C. Enucleation.