The correct answer is (c), photochemical smog.
Primary air pollutants are emitted directly from a source, such as a power plant or a car. Secondary air pollutants are formed when primary air pollutants react with other chemicals in the atmosphere. Photochemical smog is a type of secondary air pollution that is formed when sunlight reacts with nitrogen oxides and volatile organic compounds (VOCs).
Particulate matter is a mixture of solid particles and liquid droplets suspended in the air. It can be emitted from a variety of sources, including power plants, cars, and wood-burning stoves. Particulate matter can cause a variety of health problems, including respiratory problems, heart disease, and cancer.
Carbon dioxide is a greenhouse gas that is emitted from a variety of sources, including power plants, cars, and deforestation. Carbon dioxide contributes to climate change.
None of the above is a secondary air pollutant.