Which of the following is used as a nuclear fuel?

Uranium

The correct answer is option 4. Uranium.

Uranium is used as a nuclear fuel. Specifically, uranium-235 (U-235) is a fissile isotope of uranium that undergoes nuclear fission reactions. Nuclear fission is the process in which the nucleus of a heavy atom, such as uranium-235, splits into two smaller nuclei, releasing a significant amount of energy. This process is the basis for nuclear power generation in nuclear reactors, where controlled fission reactions are utilized to produce heat and generate electricity.