In IUPAC nomenclature the principal functional group decides the suffix while the longest continuous carbon chain containing that group fixes the root name. An aldehyde group, -CHO, always occupies a terminal carbon and is denoted by the suffix '-al', with the carbonyl carbon automatically taking position 1. A continuous chain of four carbons gives the root 'but-', and combining the saturated parent 'butane' with the aldehyde suffix yields butanal. Butan-1-ol is wrong because that name belongs to an alcohol (-OH), not an aldehyde. Butanone is incorrect since it describes a ketone with the carbonyl on an internal carbon. Butanoic acid is wrong as -oic acid denotes a carboxylic acid (-COOH) rather than -CHO. This naming logic follows the NCERT rules for selecting the senior characteristic group and the longest parent chain. Sanity check: the -CHO group contributes one carbon, so a four-carbon aldehyde is CH_3CH_2CH_2CHO, consistent with butanal.