Functional groups are the set of atoms in a compound that's responsible for the chemical properties of the compound. the functional groups for the given compounds are as follows methanoic acid = carboxylic group chloroethane = haloalkane propanone = ketone option A is incorrect as only one of the given compounds is a carboxylic acid, others have other functional groups option C is incorrect as they have three different functional groups as stated above option B is incorrect as its not a homologous series. Homologous series are a set of compounds with same functional group that differs by a CH₂ group. since the functional groups are different it does not belong to a homologous series as well .
correct option is D. none of these as none of the other options are correct
