Why are the given compounds in a homologous series?
Because they can all be described by the general formula $\ce{CH3-(CH2)_n-OH}$. The only varying parameter in this case is the number of middle $-\ce{CH2}- $ groups.

So while the the number of middle $-\ce{CH2}- $ groups may vary $\left(n = 0,1,2,\ldots\right)$ they all fit in this general formula.

The formula $\ce{CH3-(CH2)_n-OH}$ tells you that:


The chain starts with a $\ce{CH3}-$ group
The chain ends with an $-\ce{OH}$ group
Contains a variable number, $n$, of $-\ce{CH2}-$ groups in between.