What characterizes an alkene?

An alkene is characterized by the presence of at least one double bond between carbon atoms. This structural feature distinguishes alkenes from other types of hydrocarbons. Alkenes have the general formula CnH2n, which indicates that they contain fewer hydrogen atoms compared to saturated hydrocarbons (like alkanes) that have only single bonds. This unsaturation gives alkenes unique chemical properties, such as the ability to undergo addition reactions.

In contrast to the other options, option A describes saturated hydrocarbons, which contain only single bonds and no double bonds. Option C mentions compounds with a carbonyl group, which refers to ketones or aldehydes, not alkenes. Option D refers to alcohols, which include hydroxyl groups (-OH), rather than any double bonds in their structure. Therefore, the correct characterization of an alkene is indeed one that highlights the presence of a double bond.