Correct Option: D  [ Nitrogen dioxide ]
Remark:
Nitrogen dioxide (NO₂) is a major cause of acid rain. When released into the atmosphere from vehicle exhausts and industrial emissions, it reacts with water vapor to form nitric acid (HNO₃). Along with sulfur dioxide forming sulfuric acid, it contributes to acid rain, which damages vegetation, aquatic life, and buildings.
- Carbon dioxide: Causes global warming, not acid rain.
- Carbon monoxide: A toxic gas but not acidic.
- Oxygen: Essential for respiration, not a pollutant.