What is the functional group present in carboxylic acids?

Carboxylic acids are characterized by the presence of the carboxyl functional group, which is represented as -COOH. This group is unique because it consists of a carbon atom double-bonded to an oxygen atom (which forms the carbonyl part of the group) and also bonded to a hydroxyl group (-OH). This combination of functional groups gives carboxylic acids their acidic properties, as they can release a proton (H+) into solution, thus acting as acids.

The other functional groups listed do not pertain to carboxylic acids. The hydroxyl group (-OH) is found in alcohols, the amino group (-NH2) is characteristic of amines, and the aldehyde group (-CHO) is specific to aldehydes. Hence, the presence of the -COOH group is what definitively identifies carboxylic acids and is key to understanding their chemical properties and reactions.