Which of the following is used as an anaesthetic agent?

N₂O

The correct answer is option 3. N₂O.

Among the options provided, the anaesthetic agent is \(N2_O\) (Nitrous Oxide). \(N_2O\) commonly known as laughing gas, is a gas with anaesthetic and analgesic properties. It is used as an anaesthetic agent in dentistry and minor surgical procedures, as well as for pain management during childbirth. Nitrous oxide produces a dissociative state and mild sedation, reducing pain and anxiety in patients.

The other options are:

\(CH_4\) (Methane): Methane is a simple hydrocarbon gas and is not used as an anaesthetic agent. It is primarily known for its role as a greenhouse gas and as a fuel source.

\(CO_2\) (Carbon Dioxide): Carbon dioxide is a gas that is naturally produced in the body as a waste product of metabolism. While carbon dioxide can have physiological effects, it is not used as an anaesthetic agent. In fact, elevated levels of carbon dioxide in the blood can lead to respiratory acidosis and other health complications.

\(NO_2\) (Nitrogen Dioxide): Nitrogen dioxide is a toxic gas and a major component of air pollution. It is not used as an anaesthetic agent and can cause respiratory irritation, lung damage, and other adverse health effects.

Among the options provided, nitrous oxide \((N_2O)\) is the anaesthetic agent used for its sedative and analgesic properties in medical and dental procedures. It is distinct from methane \((CH_4)\), carbon dioxide \((CO_2)\), and nitrogen dioxide \((NO_2)\), which are not used for anaesthesia and may have other physiological effects or environmental implications.