Chemistry - STRUCTURAL ISOMERISM Question with Solution | TestHub

(A) is an enol (pent-1-en-2-ol), which tautomerises to pentan-2-one. (B) is cyclohex-1-en-1-ol, the enol of cyclohexanone. (D) phenol tautomerises to cyclohexa-2,4-dien-1-one. (C), pent-3-en-2-ol, is an allylic alcohol : the OH is on an sp3 carbon and there is no C=O or C=C-OH arrangement, so it has no tautomer. Hence (C).