Methylbenzene is heated under reflux with alkaline $\mathrm{KMnO}{}_4$ and the mixture is then acidified. The organic product is

A benzylic carbon carrying at least one hydrogen is oxidised completely to $-\mathrm{COOH}$ by hot alkaline $\mathrm{KMnO}{}_4$. Hence the methyl group of methylbenzene becomes carboxyl, giving benzoic acid after acidification.