Which of the following gases is beneficial in the upper atmosphere because it blocks ultraviolet (UV) radiation from reaching the ground but is harmful near ground level because it causes respiratory problems in humans?

Carbon monoxide is a colorless, odorless gas that is primarily produced from burning fossil fuels. While it poses serious health risks when inhaled, it does not absorb UV radiation and therefore does not play a protective role in the atmosphere. Instead, it can lead to poisoning and cardiovascular issues.

Nitrogen makes up about 78% of the Earth's atmosphere and is inert under most conditions. It does not absorb UV radiation or contribute to the formation of ozone, meaning it has no beneficial effects regarding UV protection. Additionally, nitrogen is not harmful to human health at typical atmospheric concentrations.

Radon is a radioactive noble gas that can accumulate in buildings, particularly in basements. While it poses serious health risks due to its radioactive properties, it does not interact with UV radiation in the atmosphere. Thus, it does not have any protective effect against UV radiation.