Alkenes are unsaturated hydrocarbons characterized by the presence of at least one carbon-carbon double bond. The most basic alkene is ethene (C\textsubscript{2}H\textsubscript{4}), which features a C=C double bond.
Other options:
(a) Ethane (C\textsubscript{2}H\textsubscript{6}) is an alkane, defined by its exclusively single bonds.
(c) Ethyne (C\textsubscript{2}H\textsubscript{2}) is an alkyne, distinguished by a triple bond.
(d) Benzene is an aromatic compound with a ring structure exhibiting alternating double bonds, rather than being a linear alkene.