The correct answer is option 3, Cobalt. Cobalt-60, an isotope of cobalt, is used in the treatment of cancer.
Option 1, Iodine, is not used for the treatment of cancer. Instead, radioiodine therapy is mainly used for the treatment of thyroid cancer and hyperthyroidism.
Option 2, Sodium, is not used for the treatment of cancer. Sodium is an essential mineral and plays a crucial role in various bodily functions, but it is not employed in cancer treatment.
Option 4, Uranium, is not used for the treatment of cancer. Uranium is a radioactive element that is primarily used as fuel in nuclear reactors and for making nuclear weapons. It does not have any recognized role in cancer therapy.
Therefore, the correct answer is option 3, Cobalt, specifically Cobalt-60, which is utilized in radiation therapy for cancer treatment.