Explanation

The soda-acid fire extinguisher contains a solution of sodium hydrogencarbonate (baking soda) with a small bottle of sulphuric acid; on activation the acid reacts with the hydrogencarbonate to release carbon dioxide, which smothers the fire. Nitric acid, sodium carbonate and potassium carbonate variants are not used in this extinguisher. The stored key B has been corrected per UPSC's official answer key.