Option 1: Uranium - Uranium is not used in the treatment of cancer. It is primarily used as fuel for nuclear reactors and in the production of nuclear weapons.

Option 2: Cobalt - Cobalt-60, an isotope of cobalt, is used in the treatment of cancer. It emits gamma rays, which can kill cancer cells. Cobalt therapy, also known as cobalt-60 therapy or gamma knife surgery, is a form of radiation therapy used for the treatment of tumors in different parts of the body.

Option 3: Sodium - Sodium is not used in the treatment of cancer. It is an essential mineral that plays a role in regulating fluid balance in the body but does not have any direct application in cancer treatment.

Option 4: Iodine - Iodine-131, an isotope of iodine, is sometimes used in the treatment of certain types of thyroid cancer. It is taken up by the thyroid gland and emits beta particles that target and destroy cancer cells in the thyroid.

Therefore, the correct answer is option 2: Cobalt.