Which of the following gases helps form the earth's protective shield?

Stratospheric ozone plays a crucial role in forming the Earth’s protective shield by absorbing the majority of the sun's harmful ultraviolet (UV) radiation. This layer of ozone is located in the stratosphere, approximately 10 to 30 miles above the Earth's surface. Without this protective shield, the intense UV radiation would lead to increased rates of skin cancer, cataracts, and other health issues, as well as harm to ecosystems and wildlife.

The other gases listed do not provide this sort of protective function. Methane is a potent greenhouse gas that contributes to climate change but does not absorb UV radiation. Radon is a naturally occurring radioactive gas associated with indoor air quality issues, and carbon dioxide is known for its role in climate change through the greenhouse effect, but it does not directly protect the Earth from UV radiation. Therefore, stratospheric ozone is unique in its ability to safeguard the planet from harmful solar radiation.