The correct answer is B) ozone. The gas that exists in the stratosphere and forms a protective shield that helps to protect the environment from the harmful effects of ultraviolet radiation is ozone (O3). Ozone is a naturally occurring gas that is concentrated in the upper atmosphere, known as the stratosphere. This ozone layer acts as a shield, absorbing and filtering out a significant portion of the sun's harmful ultraviolet radiation, which can be damaging to living organisms and the environment. The other answer choices are incorrect because: A) Nitrogen (N2) is the most abundant gas in the Earth's atmosphere, but it does not form a protective shield against ultraviolet radiation. C) Oxygen (O2) is also a major component of the atmosphere, but it does not have the same protective properties as ozone. D) Radon (Rn) is a naturally occurring radioactive gas, but it is not present in the stratosphere and does not play a role in protecting the environment from ultraviolet radiation.