Which type of compound includes a hydroxyl group (-OH)?

The type of compound that features a hydroxyl group (-OH) is an alcohol. Alcohols are characterized by the presence of this functional group, which is attached to a carbon atom. This hydroxyl group is key to the properties of alcohols, influencing their solubility, reactivity, and interactions with other molecules.

For instance, the presence of the hydroxyl group in alcohols allows them to engage in hydrogen bonding with water, making them generally soluble in water, unlike many other organic compounds that lack this functional group. This property is significant in various chemical reactions and biological systems.

While aldehydes, ketones, and carboxylic acids are also important classes of compounds in organic chemistry, they contain different functional groups that do not include the -OH group. Aldehydes contain a carbonyl group (C=O) at the end of the carbon chain, ketones have a carbonyl group within the carbon chain, and carboxylic acids include a carbonyl group attached to a hydroxyl group, but in a different context that creates a -COOH group. Thus, the defining feature of alcohols is the hydroxyl group, confirming that they are indeed the correct answer in this context.