The most common substance responsible for pollution is

Answer: (a) Smoke

Smoke (from fuels, stoves, factories and vehicles) is the most common pollutant.

The idea

Smoke is a cloud of very fine solid and liquid particles left when something burns incompletely. Every fire makes it: the kitchen stove, a factory, a vehicle, a burning field. That is why it is the most widespread pollutant.

Fuel burns with too little air → Tiny unburnt carbon particles form → They stay suspended in air → Breathed deep into the lungs

Why the others are wrong

(b) CO₂ is a greenhouse gas; it was not classed as a common pollutant then.

(c) SO₂ is a major pollutant but less widespread.

(d) CO is common in traffic but not as widespread as smoke.

Also know

In today’s terms smoke is particulate matter (PM10 and PM2.5), still the pollutant linked to the most deaths from air pollution. Smoke from cooking on wood and dung inside the house is a major health risk for women and children; the Ujjwala scheme for LPG connections addresses it.