The correct answer is option 'A', oxides of sulphur.

Air pollution refers to the presence of harmful substances in the atmosphere that can have negative effects on human health, the environment, and the overall well-being of living organisms. Air pollutants can be classified into several categories based on their origin and composition. Some of the major constituents of air pollutants include:

1. Oxides of Sulphur (SOx):
- Oxides of sulphur, mainly sulphur dioxide (SO2) and sulphur trioxide (SO3), are major air pollutants.
- SO2 is primarily emitted from the burning of fossil fuels, such as coal and oil, in power plants, industries, and residential heating.
- SO3 is formed by the oxidation of SO2 in the atmosphere.
- These pollutants contribute to the formation of acid rain, smog, and respiratory problems.

2. Oxides of Nitrogen (NOx):
- Oxides of nitrogen, particularly nitrogen dioxide (NO2) and nitric oxide (NO), are another significant group of air pollutants.
- They are released from the combustion of fossil fuels in vehicles, power plants, and industrial processes.
- NOx emissions are responsible for the formation of ground-level ozone (smog) and contribute to respiratory issues, cardiovascular problems, and the formation of fine particulate matter (PM2.5).

3. Carbon Monoxide (CO):
- Carbon monoxide is a colorless and odorless gas produced by incomplete combustion of carbon-based fuels, such as gasoline, natural gas, and wood.
- It is emitted from vehicles, industrial processes, and residential heating systems.
- CO is highly toxic and can cause health problems, especially in enclosed spaces with poor ventilation.

4. Hydrogen Sulphide (H2S):
- Hydrogen sulphide is a gas with a strong odor of rotten eggs.
- It is released from industrial activities, wastewater treatment plants, and natural processes like volcanic eruptions and the decay of organic matter.
- H2S is highly toxic and can cause respiratory irritation and other health issues.

In conclusion, while all of the mentioned options are air pollutants, the most major constituent among them is oxides of sulphur (SOx). These pollutants are primarily emitted from the burning of fossil fuels and contribute to various environmental and health problems.