What do methanoic acid (hcooh), 1-chloroethane (ch2clch3), and propanone (ch3coch3) have in common? a. they are all carboxylic acids. b. they are in a homologous series. c. they have the same functional group. d. none of these

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