Which element is used as fuel in most nuclear reactors?

Uranium-235 is the most widely used nuclear fuel in reactors worldwide. Option C is correct: U-235 is a fissile material, meaning it can sustain a nuclear chain reaction when bombarded with slow neutrons. Natural uranium contains only 0.7% U-235 and 99.3% U-238. Enriched uranium with 3-5% U-235 is used in most commercial reactors. Option A is incorrect: Plutonium-239 is produced in reactors from U-238 and is used in some fast breeder reactors and nuclear weapons, but it is not the primary fuel in most commercial reactors. It is fissile but not the main fuel used worldwide. Option B is incorrect: Thorium-232 is fertile, not fissile. India has large thorium reserves and is developing thorium-based reactors. Option D is incorrect: Radium-226 is a naturally occurring radioactive element that emits alpha particles. It is not used as nuclear reactor fuel.