Burning of fossil fuels is the main cause of release of ______ dioxide gas in air.
The major sources of air pollution are fossil fuels( coal and petroleum) and industries. Coal is largely carbon but contains some incombustible minerals, sulphur and nitrogen. Petroleum consists mainly of hydrocarbons, sulphur and nitrogen but also contains lead. Burning of fossil fuels produces oxides of carbon, nitrogen and sulphur.
So, the correct answer is 'Carbon dioxide'.