Toluene on heating with alkaline $\mathrm{KMnO}{}_4$ followed by acidification with $\mathrm{H}{}_3\mathrm{O}{}^{+}$ gives

Hot alkaline $\mathrm{KMnO}{}_4$ oxidises any alkyl side chain bearing a benzylic hydrogen completely to $-\mathrm{COOH}$. The $\mathrm{CH}{}_3$ of toluene becomes $\mathrm{COOH}$, so benzoic acid is formed after acidification.