Carbon(II)oxide, (CO), is primarily a product of the incomplete combustion of carbon-containing fuels such as natural gas, coal, wood, and oil. When there is an inadequate supply of oxygen during burning, CO is produced instead of the less harmful carbon dioxide (CO₂).

• A. photochemical smog is a type of air pollution resulting from the reaction of sunlight with nitrogen oxides and volatile organic compounds; carbon monoxide is involved in the complex cycles of reactions, but it is a primary pollutant produced by combustion, not the source of the smog itself. The primary components of photochemical smog are secondary pollutants like ozone and peroxyacetyl nitrate (PAN).
• B. respiration is a natural process in living organisms that produces carbon dioxide (CO₂) as a byproduct, not carbon monoxide (CO). CO₂ is an essential component of the carbon cycle and not typically considered an air pollutant in the same toxicological sense as CO (though it is a major greenhouse gas).
• D. decomposition of sewage involves biological processes that can produce various gases, such as methane (CH₄) or hydrogen sulfide, but not significant amounts of carbon monoxide as an air pollutant. The correct option is C. combustion of fuel.