Which of the following compounds belongs to the same homologous series?

$\mathit{HCOOH},\mathit{ C}{H}_3\mathit{COOH},\mathit{ C}{H}_{3}C{H}_2\mathit{COOH}$ compounds belong to the same homologous series.
A homologous series is a group of substances that have the same chemical characteristics as they have the same functional groups, which differ by $C{H}_2$ unit. 

Inorganic compounds with the same general formula have been found to include carbon chains of various lengths. Formic acid$\left(\mathit{HCOOH}\right)$, acetic acid $\left(C{H}_3\mathit{COOH}\right)$and propanoic acid$\left(C{H}_{3}C{H}_2\mathit{COOH}\right)$ are homologous series which differ in $C{H}_2$unit.