Tollens' reagent is an ammoniacal silver nitrate solution containing the diamminesilver(I) ion, which is a mild oxidising agent specific to easily oxidised carbonyl compounds. Aldehydes reduce the silver(I) complex to metallic silver, depositing a bright silver mirror on the tube wall, while being themselves oxidised to carboxylate ions. Propanal, an aldehyde, therefore gives a positive silver mirror test. Neutral ferric chloride gives a violet or coloured complex only with phenols, so propanal shows no reaction with it. Option Phenol is wrong because it does not reduce Tollens' reagent but does give a colour with neutral ferric chloride, the opposite behaviour. Option Propan-2-ol is wrong because secondary alcohols are not oxidised by the mild Tollens' reagent and give no mirror. Option Propanone is a ketone, which lacks the hydrogen on the carbonyl carbon needed for the gentle oxidation and so fails the Tollens' test. This selective oxidation is the NCERT basis for distinguishing aldehydes from ketones and alcohols. Plausibility check: only the aldehydic C-H bond permits oxidation to a carboxylic acid under such mild conditions, uniquely identifying propanal.