The constituent of air that acts as a diluent is

Correct answer: Nitrogen

Explanation: Air is about 78% nitrogen, which is largely inert under normal conditions. It dilutes oxygen, moderating combustion and preventing rapid burning or oxidation in the atmosphere.

• Nitrogen — Correct. Main diluent of atmospheric oxygen.
• Carbon(IV) oxide — Present in very small amounts (~0.04%); not the principal diluent.
• Noble gases — Trace amounts; too little to act as the main diluent.
• Oxygen — The reactive component being diluted, not the diluent.