Explanation:

Cobalt-60 ($^{60}\text{Co}$) is a radioactive isotope widely used in radiation therapy (radiotherapy) for cancer treatment. It emits high-energy gamma rays that can destroy cancerous cells. Option D, Cobalt-60, is the correct answer. Option A, Manganese-53 ($^{53}\text{Mn}$), is a stable isotope of manganese and has no known therapeutic use in cancer. Option B, Uranium-234 ($^{234}\text{U}$), is a radioactive isotope of uranium, primarily used in nuclear power and weapons, not cancer therapy. Option C, Plutonium-238 ($^{238}\text{Pu}$), is a radioactive isotope used as a power source for spacecraft and pacemakers, but not for cancer treatment due to its high toxicity and alpha emission.