Distinguishing an aldehyde from a ketone relies on the fact that aldehydes are readily oxidised while ketones resist mild oxidation because they lack a hydrogen on the carbonyl carbon. Tollens' reagent, an ammoniacal silver nitrate solution, oxidises the aldehyde group to a carboxylate and is itself reduced, depositing metallic silver as a bright mirror; propanone gives no reaction and no mirror, so this reagent cleanly separates the two. 2,4-dinitrophenylhydrazine is wrong because it forms orange precipitates with both aldehydes and ketones, providing no distinction. Sodium bisulphite also adds to both many aldehydes and methyl ketones, so it fails to discriminate here. Bromine water can be decolourised by various reducing or unsaturated species and is not a selective carbonyl test. This selective oxidation is the NCERT basis for the Tollens' silver mirror test and a routine JEE identification question. As a plausibility check, only the aldehyde supplies the C-H bond needed for oxidation to the acid, matching the observation that solely propanal reacts.