Since both the compounds have the same principal functional group, they can't be functional isomers. Obviously they are different compounds. Now options A and C are ruled out. Further the substituent ketone in first compound and aldehyde in the second compound have the same locant, so the given compounds cannot be positional isomers. So the compounds must be Chain Isomers. Further, we can just say, as there is difference in the chain length, the compounds are chain isomers. But the answer is incorrect.
The solution in my book, for this problem states, since compounds have different functional groups ketone in first compound and aldehyde in the second compound, they are functional isomers.
Now, I am totally confused. Please explain how to attend this question. Is the solution incorrect?