Explanation

Cobalt-60 is the radioisotope widely used in the treatment of cancer through external-beam teletherapy (the 'cobalt bomb'), because it emits penetrating gamma rays that destroy malignant cells. Uranium is a nuclear fuel, not a therapeutic isotope. Iodine-131 is used for thyroid disorders (diagnosis and therapy of thyroid cancer), not for general cancer radiotherapy. Fluorine-18 is a positron emitter used in PET scans for imaging, not for treatment. Hence cobalt is the correct answer.