The correct answer is (c), Radon.
Noble gases are elements in group 18 of the periodic table. They are all gases at standard temperature and pressure and are very unreactive. Helium, argon, and neon are all present in air, while radon is a radioactive gas that is not found in air.
Helium is the second most abundant element in the universe, after hydrogen. It is a colorless, odorless, and tasteless gas that is lighter than air. Helium is used in balloons, blimps, and other applications where a light gas is needed.
Argon is the third most abundant element in the Earth’s atmosphere, after nitrogen and oxygen. It is a colorless, odorless, and tasteless gas that is used in lighting, welding, and other applications where an inert gas is needed.
Neon is a colorless, odorless, and tasteless gas that is used in signs, lasers, and other applications where a bright light is needed.
Radon is a radioactive gas that is found in the Earth’s crust. It is a colorless, odorless, and tasteless gas that can be harmful to human health. Radon is the second leading cause of lung cancer in the United States.