The correct answer is (d) Cobalt-60.
Cobalt-60 is a radioactive isotope of cobalt that is used in the treatment of blood cancer. It is a beta emitter, which means that it emits beta particles when it decays. Beta particles are high-energy electrons that can damage DNA, which can kill cancer cells.
Iodine-131 is a radioactive isotope of iodine that is used in the treatment of thyroid cancer. It is a gamma emitter, which means that it emits gamma rays when it decays. Gamma rays are high-energy photons that can damage DNA, but they are not as effective at killing cancer cells as beta particles.
Sodium-24 is a radioactive isotope of sodium that is used in the treatment of bone cancer. It is a beta emitter, but it is not as effective at killing cancer cells as cobalt-60.
Phosphorus-32 is a radioactive isotope of phosphorus that is used in the treatment of leukemia. It is a beta emitter, but it is not as effective at killing cancer cells as cobalt-60.