The gas leaked from a storage tank of the Union Carbide plant in Bhopal gas tragedy was: (1) Methylamine (2) Ammonia (3) Phosgene (4) Methylisocyanate

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Tha gas leaked was methyl isocyanate (MIC). It occurred on the night of 2–3 December 1984 at the Union Carbide India Limited (UCIL) pesticide plant in Bhopal, Madhya Pradesh. Over 500,000 people were exposed to the gas and other chemicals.

The gas that leaked in bhopal gas tragedy was Methylisocyanate.
The chemical process employed in the Bhopal plant had methylamine reacting with phosgene to form MIC, which was then reacted with 1-naphthol to form the final product, carbaryl.

In November 1984, most of the safety systems were not functioning and many valves and lines were in poor condition. In addition, several vent gas scrubbers had been out of service as well as the steam boiler, intended to clean the pipes. Another issue was that Tank 610 contained 42 tons of MIC, more than safety rules allowed for. During the night of 2–3 December 1984, water entered a side pipe that was missing its slip-blind plate and entered Tank E610 which contained 42 tons of MIC. A runaway reaction started, which was accelerated by contaminants, high temperatures and other factors. The reaction was sped up by the presence of iron from corroding non-stainless steel pipelines. The resulting exothermic reaction increased the temperature inside the tank to over 200°C (392°F) and raised the pressure. This forced the emergency venting of pressure from the MIC holding tank, releasing a large volume of toxic gases. About 30 metric tons of methyl isocyanate (MIC) escaped from the tank into the atmosphere in 45 to 60 minutes.