Which of the following is an air pollutant:
A) N2
B) CO2
C) O2
D) CO

To determine which of the following is an air pollutant, let's analyze each option:

A) \[ \text{N}_2 \] (Nitrogen Gas): This is the major component of Earth's atmosphere, making up about 78% of it. It is not considered an air pollutant because it is naturally occurring and doesn't have harmful effects on health or the environment in its standard concentrations.

B) \[ \text{CO}_2 \] (Carbon Dioxide): While \[ \text{CO}_2 \] is a natural component of the atmosphere and is produced by processes like respiration and combustion, its elevated levels due to human activities (e.g., burning fossil fuels) contribute to global warming and climate change. In the context of air pollution, \[ \text{CO}_2 \] is considered a pollutant when it is present in excess.

C) \[ \text{O}_2 \] (Oxygen Gas): This is essential for respiration and is also a major component of the atmosphere. It is not an air pollutant because it is necessary for life and does not harm health or the environment under normal atmospheric conditions.

D) CO (Carbon Monoxide): This is a colorless, odorless gas that is produced from incomplete combustion of carbon-containing fuels. It is a harmful air pollutant because it can interfere with the body's ability to transport oxygen and can cause serious health problems, including cardiovascular and respiratory issues.

So, the correct answer is:

**D) CO**