The correct answer is Option C — Calcium. Calcium is a common alkaline earth metal (atomic number 20) that has no fissile or fertile properties. It cannot sustain a nuclear chain reaction and is therefore NOT used as nuclear fuel. Calcium is used in biological systems (bones, teeth) and in metallurgy. Option A is incorrect: Uranium is the most widely used nuclear fuel. U-235 is fissile (can sustain chain reaction with slow neutrons) and U-238 is fertile (can be converted to Pu-239). Natural uranium contains 99.3% U-238 and 0.7% U-235. Option B is incorrect: Plutonium-239 is a fissile material produced in reactors from U-238. It is used as fuel in fast breeder reactors. India's PFBR at Kalpakkam will use Pu-239 as fuel. It was also used in the Fat Man bomb dropped on Nagasaki. Option D is incorrect: Not all are fuels because Calcium cannot be used as nuclear fuel. Nuclear fuels include fissile (U-235, Pu-239, U-233) and fertile (Th-232, U-238) materials.
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