Chemistry - STRUCTURAL ISOMERISM Question with Solution | TestHub

(A) is but-2-en-2-ol, the enol of butan-2-one. (B) phenol tautomerises to cyclohexa-2,4-dien-1-one. (D) is cyclopent-1-en-1-ol, the enol of cyclopentanone. (C), prop-2-en-1-ol (allyl alcohol), has its $\mathrm{OH}$ on an sp3 carbon and no $\mathrm{C}=\mathrm{O}$ or C=C-OH arrangement, so it has no tautomer. Hence (C).