Which of the following is NOT a homologous series?

C2H4, C2H6, C3H6, C4H8 is NOT a homologous series, because the set mixes an alkane (C2H6) with alkenes (C2H4, C3H6, C4H8); members of a true homologous series must share the same functional group and differ successively by a -CH2- unit. A homologous series is a group of organic compounds that have a similar structure and chemical properties, and each successive compound in the series differs by a CH2 unit.