The correct answer is NH\(_3\) - option C

A. HCl (Hydrogen Chloride) is a colourless, pungent gas that is highly soluble in water, forming a strong acid (hydrochloric acid), and is classified as a hydrogen halide. While the gas itself isn't acidic (it needs water), it's highly corrosive and an air pollutant, reacting with moisture to create corrosive acid aerosols.

B. N\(_2\) is a neutral gas and not an alkaline gas because it's very unreactive and doesn't readily donate or accept protons or electron pairs when dissolved in water, showing no effect on litmus paper.

C. NH\(_3\) is an example of an alkaline gas. Ammonia (NH\(_3\)), which is a colourless gas with a pungent smell, forms basic (alkaline) solutions when dissolved in water, producing hydroxide ions.

D. NO\(_2\) is an acidic gas and not an alkaline gas because it reacts with water in the atmosphere to form nitric acid (HNO\(_3\) and nitrous acid(HNO\(_2\)), thereby forming acid rain.

:. The answer is NH\(_3\) - option C