Q.The major air pollutant is :

All the listed gases are major air pollutants.

Major gaseous air pollutants, mostly produced by the combustion of fossil fuels, include:

• Carbon monoxide (CO): a toxic gas from incomplete combustion
• Oxides of nitrogen (NOx): contribute to smog and acid rain
• Oxides of sulphur (SOx, mainly SO2): cause acid rain and respiratory problems …

CO₂ is the odd one out — not a conventional pollutant — option (b).

From NCERT Class 11 Chemistry (Environmental Chemistry):

• NO₂: toxic oxide of nitrogen, causes acid rain and photochemical smog — a pollutant.
• O₃ at ground level: a secondary pollutant (photochemical smog), harmful to lungs.
• C₆H₆ (benzene): a toxic, carcinogenic hydrocarbon pollutant. …