Two isomers 'A' and 'B' with molecular formula \( \mathrm{C}_4\mathrm{H}_8 \) give different products on oxidation with \( \mathrm{KMnO_4} \) in acidic medium. Isomer 'A' on reaction with \( \mathrm{KMnO_4} / \mathrm{H^+} \) results in effervescence of a gas and gives ketone. The compound 'A' is: