Which of the following is not a member of a homologous series?
CH3−CH2−CH2−Cl|CH2
What do you mean by a homologous series?
In chemistry, a homologous series is a series of compounds with the same general formula, usually varying by a single parameter. In the case of organic chemistry, that parameter is the length of the carbon chain.
So, now you can take a look at the options. Option (a), (b) ad (c) are from the same series, only having a difference of - CH2. Option (d) does not belong to the same series as other options because it has one chlorine attached to it.