The correct answer is C. Ozone (O3).
A secondary pollutant is a pollutant formed when primary pollutants react with other chemicals in the atmosphere. Ozone is a secondary pollutant that is formed when nitrogen oxides (NOx) and volatile organic compounds (VOCs) react in the presence of sunlight.
CO2, CO, and SO2 are primary pollutants. Primary pollutants are pollutants that are emitted directly into the atmosphere from a source, such as a power plant or a car.
CO2 is a greenhouse gas that contributes to climate change. CO is a colorless, odorless gas that is poisonous to humans. SO2 is a gas that can cause acid rain.
Ozone is a gas that is found in the stratosphere, where it protects the Earth from harmful ultraviolet radiation. However, ozone in the troposphere, the lowest layer of the atmosphere, is a pollutant that can cause respiratory problems and other health issues.