Which compound is primarily known for being an unsaturated hydrocarbon?

An unsaturated hydrocarbon is defined by the presence of one or more double or triple bonds between carbon atoms, which allows for additional atoms to bond with the carbon skeleton. Among the options provided, alkenes specifically fit this definition because they contain at least one carbon-carbon double bond, making them unsaturated.

In contrast, alkanes are saturated hydrocarbons that only contain single bonds between carbon atoms, which means they are fully "saturated" with hydrogen atoms. Alcohols, while they may contain carbon and hydrogen, have a hydroxyl (-OH) group that changes their classification from hydrocarbons to alcohols. Aldehydes also contain carbon-based structures but feature a carbonyl group (C=O) that shifts them out of the hydrocarbon category.

Thus, alkenes are primarily recognized as the unsaturated hydrocarbons due to their unique double bond configuration, allowing them to react differently compared to their saturated counterparts.