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Among the three main types of radioactive radiation, gamma rays have the highest penetrating power. Option C is correct: Gamma rays are electromagnetic radiation with very short wavelength and high energy. Option A is incorrect: Alpha particles are helium nuclei (2 protons + 2 neutrons) and have the lowest penetrating power. They have the highest ionizing power but lowest penetration. Option B is incorrect: Beta particles are fast-moving electrons or positrons. They have moderate penetrating power and can be stopped by a few millimetres of aluminium. They are more penetrating than alpha but much less than gamma. Option D is partially correct but X-rays are not radioactive decay products; they are produced by electron transitions. Gamma rays are more energetic than typical X-rays. The order is: Alpha < Beta < Gamma.
