**Core Concept**
Retinoblastoma is a malignant tumor of the retina, and metastatic disease refers to the spread of cancer cells beyond the primary site to distant organs. The treatment of metastatic disease in retinoblastoma is a complex issue, involving a combination of chemotherapy, radiation therapy, and targeted therapy.

**Why the Correct Answer is Right**
The treatment of metastatic retinoblastoma often involves the use of chemotherapy agents such as carboplatin, etoposide, and vincristine. These agents work by targeting rapidly dividing cells, such as cancer cells, and preventing them from reproducing. In addition, targeted therapy agents such as bevacizumab may be used to inhibit the growth of new blood vessels that feed the tumor. The goal of treatment is to control the growth of the tumor and alleviate symptoms.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because chemotherapy is just one aspect of the treatment of metastatic retinoblastoma. While chemotherapy is a crucial component, it is not the only treatment option.
**Option B:** This option is incorrect because radiation therapy is not typically used as a primary treatment for metastatic retinoblastoma. However, it may be used in certain situations to relieve symptoms or control tumor growth.
**Option C:** This option is incorrect because there is no evidence to support the use of immunotherapy as a primary treatment for metastatic retinoblastoma.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the treatment of metastatic retinoblastoma is highly individualized and depends on various factors, including the extent of disease, the patient's overall health, and the presence of any genetic mutations.

**Correct Answer: D. Chemotherapy.