Buta-1,3-diene is heated with ethene to form a cyclic hydrocarbon Y. Y is oxidised with hot concentrated $\mathrm{KMnO}{}_4$ followed by acidification. The final organic product is

The Diels-Alder reaction of buta-1,3-diene with ethene gives cyclohexene (Y). Hot concentrated permanganate cleaves the C=C completely, and each carbon (each is a $=\mathrm{CH}$) is oxidised to $\mathrm{COOH}$. The ring opens to the chain $\mathrm{HOOC}-(\mathrm{CH}{}_2){}_4-\mathrm{COOH}$, hexanedioic acid. Cold alkaline permanganate would instead give the diol.