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The correct answer is Option C — Molecule. A molecule is the smallest unit of a substance that can exist independently in a free state and retain all the properties of that substance. Option A (Electron): An electron is a subatomic particle, NOT a unit of substance. It cannot exist independently as a substance. Option B (Atom): An atom CAN exist independently for noble gases (like He, Ne, Ar), but for most substances, the molecule is the basic unit. An atom is a unit of an element, not a compound. Option D (Ion): An ion is a charged atom/molecule. It cannot exist freely in a neutral state.
