Match List I with List II.
              List I                                                         List II.
A. Propanamine and N-Methylethanamine          I. Metamers
B. Hexan-2-one and Hexan-3-one                        II. Positional isomers
C. Ethanamide and Hydroxyethanimine             III. Functional isomers
D. o-nitrophenol and pnitrophenol                     IV. Tautomers
Choose the correct answer from the options given below :-

• A. Propanamine and N-Methylethanamine $$\rightarrow$$ III. Functional isomers
  • Propanamine is a primary ($$1^{\circ}$$) amine (functional group: $$-\text{NH}_2$$), while N-methylethanamine is a secondary ($$2^{\circ}$$) amine (functional group: $$-\text{NH}-$$). Since primary and secondary amines contain different functional groups, they are functional isomers.
• B. Hexan-2-one and Hexan-3-one $$\rightarrow$$ I. Metamers
  • Both are ketones sharing the same polyvalent functional group ($$-\text{CO}-$$). However, they differ in the alkyl groups attached to either side of the carbonyl carbon (methyl & butyl vs. ethyl & propyl), making them metamers.
• C. Ethanamide and Hydroxyethanimine $$\rightarrow$$ IV. Tautomers
  • These two structures exist in a dynamic equilibrium via the migration of a proton ($$\text{H}^+$$) and the shifting of a double bond (amide-imidol tautomerism). Thus, they are tautomers.
• D. o-Nitrophenol and p-Nitrophenol $$\rightarrow$$ II. Positional isomers
  • Both possess the exact same functional groups on a benzene ring, but the nitro ($$-\text{NO}_2$$) group is located at different positions (ortho vs. para) relative to the hydroxyl ($$-\text{OH}$$) group. Therefore, they are positional isomers.

Combining these correct matches gives: A-III, B-I, C-IV, D-II.

The correct option is D.