Question

What do methanoic acid (HCOOH), 1-chloroethane (CH2ClCH3), and propanone (CH3COCH3) have in common?

Answer 1

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