The correct answer is A. U-235.
Uranium-235 is a fissile isotope of uranium, meaning that it can be split by neutrons to release energy. This process is called nuclear fission. When a uranium-235 atom is split, it releases two or three neutrons. These neutrons can then split other uranium-235 atoms, releasing more neutrons, and so on. This chain reaction can be used to generate electricity in a nuclear power plant or to create a nuclear weapon.
Uranium-238 is not fissile, but it can be converted into plutonium-239, which is fissile. Plutonium-239 is used in nuclear weapons and some nuclear power plants.
Uranium-234 is a radioactive isotope of uranium. It is not fissile, but it decays into uranium-238 over time.
Uranium-245 is a very rare isotope of uranium. It is not fissile, and it decays into thorium-231 over time.