Isomers are compounds with the same molecular formula but different structures. Tautomers are structural isomers that interconvert via proton transfer (e.g., keto-enol). Functional isomers have different functional groups. Geometrical isomers differ in spatial arrangement around a double bond. Identical compounds are the same molecule. Metamers differ in alkyl group distribution.

For the given structures: II and I are functional isomers (ketone vs. aldehyde), II and III are tautomers (keto-enol forms), II and IV are identical (same connectivity).

Final answer: Functional, tautomer, identical