**Core Concept**
Radioactive iodine is a treatment option for hyperthyroidism, particularly in conditions like Graves' disease, where it selectively targets and destroys thyroid tissue. This is based on the principle of radioactive decay, where iodine-131 emits radiation that damages thyroid cells.

**Why the Correct Answer is Right**
Radioactive iodine is taken up by the thyroid gland through the sodium-iodide symporter, which is selectively expressed in thyroid follicular cells. Once inside, the radioactive iodine emits beta particles that damage the thyroid tissue, leading to a reduction in thyroid hormone production. This treatment is particularly useful in Graves' disease, as it can provide a definitive cure in many cases.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the question. We are looking for a specific condition where radioactive iodine can be used, not a general statement about its use.

**Option B:** This option is incorrect because radioactive iodine is not typically used to treat hypothyroidism. While it can be used to treat hyperthyroidism, the goal is to reduce thyroid hormone production, not increase it.

**Option C:** This option is incorrect because radioactive iodine is not used to treat thyroid cancer. While it can be used to treat hyperthyroidism, its use in cancer treatment is typically limited to certain types of thyroid cancer that are refractory to other treatments.

**Clinical Pearl / High-Yield Fact**
Remember that radioactive iodine is a definitive treatment for Graves' disease, but it may take several months to a year or more to achieve a euthyroid state.

**Correct Answer:** A. Graves' disease.