The correct answer is option 2: alkenes.

C4H8 is a hydrocarbon molecule that consists of 4 carbon atoms and 8 hydrogen atoms. It belongs to the homologous series of alkenes. Homologous series are a group of organic compounds that have the same general formula, similar chemical properties, and show a gradual increase in molecular weight.

Alkenes are hydrocarbons that contain at least one carbon-carbon double bond. The general formula for alkenes is CnH2n, where n represents the number of carbon atoms in the molecule. In the case of C4H8, there are 4 carbon atoms, so it fits the general formula CnH2n, making it an alkene.

Option 1, alkanes, are hydrocarbons that only have single bonds between the carbon atoms. Option 3, alkynes, are hydrocarbons that contain at least one carbon-carbon triple bond. Option 4, cycloalkenes, are cyclic hydrocarbons that contain at least one carbon-carbon double bond.

Therefore, based on the molecular formula and the general formula, C4H8 belongs to the homologous series of alkenes.