Explanation:

- Laughing Gas is a common name for a particular gas used for its anesthetic effects, especially in dentistry. It is known for inducing a state of euphoria and laughter when inhaled.

- Option 1: Sulphur dioxide

- Sulphur dioxide is a gas with a pungent, suffocating smell. It is primarily known for its role in air pollution and as an industrial byproduct. It is not associated with inducing laughter or euphoria.

- Option 2: Nitrogen oxide

- While this term can technically refer to several oxide compounds of nitrogen, the correct term for laughing gas is nitrous oxide (N2O). Nitrous oxide is used for sedation in medical settings and is the actual compound associated with the term "laughing gas."

- Option 3: Carbon dioxide

- Carbon dioxide is a natural byproduct of respiration and is used in carbonated beverages. It has no significant euphoric or anesthetic properties that would link it to the term "laughing gas."

- Option 4: Carbon monoxide

- Carbon monoxide is a toxic gas that can cause serious health effects and even death in high concentrations. It is certainly not linked to laughter or safe medical use.

- Correct Answer: Option 2 (Nitrogen oxide, specifically nitrous oxide)