Correct Answer: (c) Actinium
Solution:The noble gases make a group of chemical elements with similar properties under standard conditions. They are all odourless, colourless, monatomic gases with very low chemical reactivity. These gases do not undergo chemical reactions under certain given conditions. The six noble gases that occur naturally are Helium (He), Neon (Ne), Argon (Ar), Krypton (Kr), Xenon (Xe), and the radioactive Radon (Rn).