The gas, which is emitted in the paddy fields and increases the earth's temperature is

1. A

   Nitrogen

   Nitrogen gas is inert in the greenhouse sense at this exam level and is not the methane-from-paddy story. Flooded soils emit methane, not N2 as the warming gas named here.

2. B

   Carbon dioxide

   Carbon dioxide is a greenhouse gas but the distinctive emission from anaerobic paddy soils is methane from methanogens, not CO2 as the intended answer.

3. C

   Carbon monoxide

   Carbon monoxide is a pollutant from incomplete combustion, not the principal rice-field greenhouse gas in this question.

4. D

   Methane

   Methane is produced by anaerobic methanogenic archaea in waterlogged paddy soils and is a potent greenhouse gas that raises Earth’s temperature. That is why (d) is the official key.

Summary. Official key is (d) methane. Flooded rice fields are a major anthropogenic CH4 source. CO2 and CO are different stories; N2 is not the gas meant here.