Concept:Nuclear fuel is a radioactive material used to sustain a fission chain reaction in a reactor.Explanation:Uranium, especially U-235, is the most widely used nuclear fuel in power reactors.It undergoes controlled nuclear fission and releases a large amount of energy.Palladium is not a radioactive fuel.Actinium is radioactive but is not commonly used as a nuclear fuel.Thorium can be used in advanced reactors, but uranium remains the common fuel.Answer:A. uranium