Neutral ferric chloride is the classical reagent for detecting phenols. Phenols ionise to give phenoxide ions, which act as ligands and coordinate to Fe^{3+} forming coloured iron-phenoxide complexes that appear violet, blue, or green depending on the phenol. The intense violet colour therefore indicates a phenolic hydroxyl group. Option Aldehyde group is incorrect because aldehydes are detected by Tollens' or Fehling's reagent and do not form coloured complexes with ferric chloride. Option Carboxylic acid group is wrong; although some carboxylic acids give a buff precipitate with ferric chloride, they do not produce the characteristic violet colour and are better identified by effervescence with sodium bicarbonate. Option Primary amine group is incorrect because amines are detected by the carbylamine test or diazotisation, not by ferric chloride colour. This colour reaction is the NCERT-prescribed confirmatory test for phenols. The reagent must be neutral because added acid would suppress phenoxide formation and added base would precipitate ferric hydroxide, in either case destroying the diagnostic colour. Enols, which also generate a stabilised oxygen anion, can give a similar response, but among the choices offered only the phenolic group qualifies. Plausibility check: the colour depends on phenoxide acting as a chelating ligand to ferric ion, so only compounds capable of forming a phenoxide, that is phenols, give the violet coordination complex.