Multiple choice general knowledge science & technology Which of these elements signifying nobel gasses is radio active ? Argon (Ar) Radon (Rn) Krypton (Kr) Helium (He) Reveal answer Fill a bubble to check yourself B Correct answer Explanation Radon (Rn) is the only radioactive noble gas among the options. All noble gases are stable except radon, which has no stable isotopes and is radioactive due to its large atomic size and nuclear instability.