Which of the following is the first member of the homologous series of carboxylic acids?

The first member of the homologous series of carboxylic acids is methanoic acid. This organic compound has the simplest structure within this series, characterized by the presence of a carboxyl group (-COOH) attached to a single carbon atom. Its molecular formula is CH₃COOH, indicating it has only one carbon atom, which makes it the base structure from which other carboxylic acids with additional carbon atoms (like ethanoic, propanoic, and butanoic acids) are derived.

In contrast, the other options involve longer carbon chains. Ethanoic acid has two carbon atoms, propanoic acid has three, and butanoic acid has four. Each subsequent member of the series increases in one carbon atom and exhibits similar chemical properties, but methanoic acid is unique as the simplest representative of this series.