Which of the following describes alkenes?

Alkenes are classified as unsaturated hydrocarbons due to the presence of one or more carbon-carbon double bonds (C=C) in their molecular structure. This double bond reduces the number of hydrogen atoms that can bond to the carbon chain, making alkenes less saturated compared to alkanes, which only contain single bonds and are fully saturated with hydrogen atoms.

Unsaturated hydrocarbons, which include alkenes and alkynes, can undergo additional reactions, such as hydrogenation, where they can react with hydrogen to become saturated. This characteristic is fundamental to their reactivity and distinguishes them from saturated hydrocarbons, which do not have double or triple bonds.

The other options do not accurately describe alkenes. Aliphatic compounds can be either saturated or unsaturated and encompass a broader category than just alkenes. Cyclic compounds may include alkenes, but not all alkenes are cyclic, as they can also be found in straight-chain or branched forms. Thus, the definition of alkenes is best captured by their classification as unsaturated hydrocarbons, highlighting their unique chemical properties.