The Bhopal Gas Tragedy occurred due to the release of

Concept: An industrial/chemical disaster is identified by tracing the exact substance manufactured or stored at the specific plant involved, not by guessing at any generic "poisonous gas" — different industrial processes release very different chemicals, each with its own distinct hazard profile.

Application: The Bhopal Gas Tragedy (night of 2-3 December 1984) occurred at the Union Carbide India Limited (UCIL) pesticide factory in Bhopal, Madhya Pradesh. The plant manufactured the pesticide Sevin using Methyl Isocyanate (MIC) as a key intermediate chemical. Water entered a storage tank holding liquid MIC, triggering an uncontrolled exothermic reaction that built up pressure and forced a dense cloud of toxic MIC vapour to escape over the sleeping city, killing thousands and injuring hundreds of thousands more. So the gas released was Methyl Isocyanate.

Cross-check: none of the other listed gases fit the plant or the event:

• Chloro-Fluoro Carbon (CFC) is an inert refrigerant/propellant compound linked to stratospheric ozone depletion — it was never manufactured or stored at the UCIL pesticide plant.

• Carbon dioxide is a routine combustion/respiration by-product, not a chemical intermediate of pesticide manufacture, so it was not the substance whose runaway reaction caused the tragedy.

• Nitrous oxide is an anaesthetic and greenhouse gas used in medical and unrelated industrial contexts; it played no role at the UCIL plant.

Result: the gas released in the Bhopal Gas Tragedy was Methyl Isocyanate.