**Core Concept**
The question deals with the use of radioactive isotopes for brachytherapy in the treatment of eye tumors. Brachytherapy involves the placement of a radioactive source close to or within the tumor to deliver a high dose of radiation while minimizing damage to surrounding healthy tissue.

**Why the Correct Answer is Right**
Phosphorus-32 (P-32) is a beta-emitting radioactive isotope that has been used for brachytherapy in the treatment of ocular melanomas, specifically for the creation of a shield (mould) to deliver a localized dose of radiation. The beta particles emitted by P-32 have a short range and can be effectively used to destroy tumor cells while sparing surrounding tissues. The use of P-32 in a shield or mould allows for precise delivery of radiation to the tumor, reducing the risk of damage to adjacent structures.

**Why Each Wrong Option is Incorrect**
**Option A:** Iodine-131 is a radioactive isotope used in the treatment of thyroid cancer and hyperthyroidism, but it is not typically used for brachytherapy in eye tumors.
**Option B:** Gold-198 is a radioactive isotope that has been used in the past for brachytherapy, but it is not the preferred choice for creating a shield in eye tumors.
**Option C:** Strontium-90 is a beta-emitting isotope that has been used in brachytherapy, but it is not typically used for the treatment of ocular melanomas.

**Clinical Pearl / High-Yield Fact**
When considering brachytherapy for eye tumors, it is essential to choose a radioactive isotope that has a suitable half-life and energy spectrum to effectively destroy tumor cells while minimizing damage to surrounding tissues. Phosphorus-32 is a well-established choice for this purpose due to its short half-life and localized radiation effects.

**✓ Correct Answer: D. Phosphorous-32**