The functional group of a carboxylic acid is

Explanation

The carboxyl group combines a carbonyl and a hydroxyl on the same carbon atom, and this arrangement makes the hydrogen far more acidic than in an alcohol, because the resulting carboxylate ion is stabilised by delocalisation. CHO is the aldehyde group and CO between two carbons is a ketone. Acids therefore liberate carbon dioxide from carbonates, which alcohols do not.