What is the functional group in alkenes?

In alkenes, the functional group is characterized by the presence of a carbon-carbon double bond, denoted as C=C. This specific structural feature is what distinguishes alkenes from other classes of hydrocarbons, such as alkanes, which only contain single bonds between carbon atoms. The C=C bond is critical for the reactivity of alkenes, as it provides sites for various chemical reactions, such as polymerization and addition reactions, making them different from compounds containing other functional groups.

The other options represent different functional groups found in other types of organic compounds, such as alcohols, carboxylic acids, and aldehydes, but they do not pertain to alkenes. The presence of the C=C functional group is what defines the unique behavior and chemical properties associated with alkenes, emphasizing its importance in organic chemistry.