Propanone (CH₃COCH₃) is a ketone, while Propanal (CH₃CH₂CHO) is an aldehyde.
Although both compounds have the same molecular formula (C₃H₆O), they differ in the functional groups they contain: propanone has a carbonyl group (C=O) bonded to two carbon atoms (as part of a ketone), while propanal has a carbonyl group (C=O) bonded to a hydrogen atom and a carbon atom (as part of an aldehyde).
Functional isomers are compounds that have the same molecular formula but different functional groups.
Thus, Propanone and Propanal are functional isomers, making option (B) the correct answer.