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Radioactivity is a natural nuclear phenomenon discovered by Henri Becquerel in 1896. Option B is correct: Radioactivity is the spontaneous emission of radiation (alpha, beta, or gamma) from the unstable nuclei of certain elements. The nucleus emits particles or energy to become more stable. This process occurs without any external stimulus and cannot be controlled by physical or chemical means. Option A is incorrect: Emission of visible light by atoms describes phosphorescence or fluorescence, which are optical phenomena, not radioactivity. These involve electron transitions, not nuclear changes. Option C is incorrect: Absorption of X-rays by matter describes X-ray attenuation, which is completely different from radioactivity. X-rays are not emitted during radioactive decay. Option D is incorrect: Chemical decomposition of compounds is a chemical reaction involving electron bonds, not nuclear processes. Radioactivity is purely a nuclear phenomenon and does not depend on the chemical state or temperature of the substance. Marie Curie and Pierre Curie were the first to study radioactivity systematically.
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