The correct option is A HCOOCH3 and CH3COOCH3
HCOOCH3 and CH3COOCH3 are members of the same homologous series. The formula of these compounds differs by a methylene group (−CH2) group.
Note: In chemistry, a homologous series is a group or class of compounds having the same general formula and similar chemical properties in which successive members differ by −CH2 group.