Which of the gases is employed as an anaesthesia?

Correct answer: N₂O

Explanation: Nitrous oxide (N₂O), also called laughing gas, is used as an inhalational anaesthetic (often with oxygen and other agents) for short procedures and analgesia.

• N₂O — Correct. Common anaesthetic/analgesic gas.
• NO₂ — Toxic, corrosive brown gas; not an anaesthetic.
• NH₃ — Ammonia; pungent and irritating, not used for anaesthesia.
• NO — Nitric oxide; a vasodilator in medicine, not an anaesthetic gas.