Which of the following is a secondary pollutant? 

O₃

The correct answer is option 1. O_3.

Ozone (O₃) is a secondary pollutant. Secondary pollutants are not directly emitted into the atmosphere but are formed through the chemical reactions of primary pollutants with other substances in the air. In the case of ozone, it is formed through complex reactions involving precursor pollutants such as nitrogen oxides (NOₓ) and volatile organic compounds (VOCs) in the presence of sunlight. Ozone is a key component of smog and can have detrimental effects on human health and the environment.