Which of the following is NOT one of the first four members of the homologous series of alkenes?

The correct option is Hexene.

The correct option is Hexene. Hexene is correct because it directly supports the approved learning objective to recall the first four members of the homologous series of alkenes as ethene, propene, butene and pentene. This belongs to Structure and formulae of alkenes within Reactions of alkenes and alcohols (chemistry only), so the answer must use the correct organic chemistry context. The other options are incorrect when they confuse the organic family, formula type, reaction condition, product, or property being tested. Keep molecular formula, structural formula, displayed formula, and general formula distinct. Do not confuse alkanes with alkenes, saturated with unsaturated, cracking with combustion, polymers with monomers, or hydrocarbons with oxygen-containing alcohols and carboxylic acids. When formulae are used, preserve the stored notation exactly and explain the GCSE chemistry idea in words rather than using unsupported displayed-formula diagrams.