Which functional group contains a carbonyl group (C=O) attached to a hydroxyl group (–OH)?

The correct choice is the one that describes a functional group which consists of a carbonyl group (C=O) directly bonded to a hydroxyl group (–OH). This specific structure is characteristic of carboxylic acids. In carboxylic acids, the carbonyl group is located at the end of the carbon chain, and the hydroxyl group is also attached to the same carbon atom, forming the carboxyl functional group (-COOH). This unique arrangement gives carboxylic acids their distinct properties, such as their acidic nature due to the ability of the hydroxyl group to donate a proton (H+).

In contrast, aldehydes and ketones both contain carbonyl groups, but do not have the hydroxyl group attached directly to the same carbon. Aldehydes have the carbonyl at the end of the carbon chain, while ketones have it within the chain but lack the –OH group entirely. Alcohols, on the other hand, contain hydroxyl groups but do not feature a carbonyl group at all. This distinction emphasizes the unique nature of the carboxylic acid functional group.