Which gas has a characteristic pungent smell, turns red litmus paper blue and forms dense white fumes with hydrogen chloride gas?

Correct answer: NH₃ (ammonia).

Explanation: Ammonia has a sharp, pungent smell, is basic (it turns red litmus paper blue), and reacts with hydrogen chloride gas to form dense white fumes of ammonium chloride:

NH₃(g) + HCl(g) → NH₄Cl(s) (white smoke)

• N₂ and N₂O are largely odorless and do not turn red litmus blue.
• Cl₂ has a choking odor and turns moist blue litmus red then bleaches; it does not give white fumes with HCl.