Answer: B. Three items distinguish the four gases.
Ammonia turns moist red litmus paper blue (alkaline). Chlorine bleaches moist litmus paper after first turning it red — so a single piece of damp blue litmus paper distinguishes \(\text{Cl}_2\). Hydrogen burns with a 'pop' when a lighted wooden splint is brought to it. Oxygen relights a glowing wooden splint.
Items needed: red litmus paper, blue litmus paper, wooden splint = 3.
The other reagents (KMnO₄, aqueous ammonia, barium nitrate, silver nitrate, NaOH, limewater) test for ions or for \(\text{CO}_2\) — irrelevant to this set of gases.
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