Acidified KMnO4 decolorizes with alkenes or aldehydes due to oxidation. Steps:

• Empirical formula C₂H₄O gives molecular formula C₂H₄O with one degree of unsaturation: ((2×2 + 2 - 4)/2 = 1), from a double bond, ring, or functional group.
• Decolorization requires a C=C bond (oxidative cleavage) or -CHO group (to -COOH); epoxides or ethers do not react.
• Structure 1 is an epoxide (cyclic ether, ring unsaturation, no reaction).
• Structures 2 and 3 contain C=C (alkene) or -CHO (aldehyde, both oxidizable).
• Structure 4 lacks C=C or -CHO (no reaction).

Why C is correct:

• Structures 2 and 3 have oxidizable features per Baeyer-Kiliani test principles for unsaturation and carbonyls.

Why the others are wrong:

• A includes non-reactive structure 1.
• B selects only non-reactive structure 1.
• D includes non-reactive structure 4.

Final answer: C