The correct answer is B) ozone. Ozone (O3) is the gas that exists in the stratosphere and forms a protective shield that helps to protect the environment from the harmful effects of ultraviolet (UV) radiation. This ozone layer absorbs and filters out a significant portion of the sun's harmful UV radiation, which can be damaging to living organisms and the environment. The other answer choices are incorrect because radon (A) is a radioactive gas found in the Earth's crust, oxygen (C) is the most abundant gas in the atmosphere, and nitrogen (D) is the second most abundant gas in the atmosphere, but neither of these gases form a protective shield against UV radiation like ozone does.