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

Correct answer: D (Methane). Paddy fields are flooded, so the soil becomes waterlogged and oxygen-poor. In these anaerobic conditions, methanogenic archaea break down organic matter and release methane. Rice cultivation is therefore one of the largest human-linked sources of methane.

Why the others fail:

• Nitrogen is the most abundant gas in the atmosphere, but it is chemically inert and does not trap infrared radiation.
• Oxygen is not a greenhouse gas. Paddy soils consume oxygen rather than emit it.
• Carbon monoxide is an indirect pollutant that reacts with OH radicals and has a short life. It is not a major paddy emission.

Methane is a potent greenhouse gas, roughly 28 times as effective as CO2 over 100 years (IPCC AR5). For Mains, link this to India's NDC commitments and practices like Alternate Wetting and Drying and direct-seeded rice, which cut methane from fields.

Trap: Candidates pick carbon monoxide or nitrogen because both are common in air-pollution questions. The clue is 'emitted in paddy fields', which points to anaerobic decomposition, and that means methane.