Burning of crop residue is an incomplete combustion process that releases various gases. Carbon monoxide (CO) and methane (CH4) are significant products of incomplete combustion. Sulphur dioxide (SO2) is released if the biomass contains sulphur. Ozone (O3) is a secondary pollutant formed in the atmosphere, often as a result of reactions involving precursors like nitrogen oxides and volatile organic compounds, which can also be released from biomass burning, but ozone itself is not directly released from the combustion process in large quantities. However, given the options and common UPSC question patterns, all listed pollutants are often considered in the context of emissions from such activities, directly or indirectly.