What is the IUPAC name for a carboxylic acid with four carbons?

The IUPAC name for a carboxylic acid with four carbons is butanoic acid. Carboxylic acids are named based on the number of carbon atoms in the longest carbon chain that contains the carboxyl group (-COOH).

In this case, with four carbon atoms, the base name of the alkane is "butane." The presence of the carboxylic acid functional group modifies this name to "butanoic acid." The number of carbon atoms dictates the prefix "but-" indicating four carbons.

The other choices refer to acids with different numbers of carbon atoms: hexanoic acid consists of six carbons, propanoic acid has three carbons, and pentanoic acid includes five carbons. This distinction underscores why butanoic acid is the only correct name for a four-carbon carboxylic acid.