Which is an example of a secondary air pollutant?

Correct option is B

Secondary pollutants are not emitted directly. They form in the atmosphere through chemical reactions between primary pollutants and atmospheric components.
Acid rain forms when:
Sulfur dioxide (SO₂) and nitrogen oxides (NOₓ) react with water vapor to form sulfuric acid and nitric acid.
In contrast:
Carbon monoxide, sulfur oxides, and lead are primary pollutants (directly emitted).
· Therefore, acid rain is a secondary pollutant.
· Correct answer: (b).