Which homologous series has the general formula CnH2n+2?

The answer to this question is the alkane series. You can work this out using the following method: Becuase each carbon must make four bonds to complete it outer shell of electrons, there must be double the amount of hydrogen atoms for each carbon atom - plus two to complete the ends of the hydrocarbon chain. This is an example of the nth rule, in which the +2 signifies this addition of two hydrogen atoms.