The compound shown is heated with hot alkaline $\mathrm{KMnO}{}_4$ and then acidified. Which structure is the product?

The compound shown is ethylbenzene. The benzylic $\mathrm{CH}{}_2$ is oxidised and the whole side chain is degraded, with the second carbon lost as $\mathrm{CO}{}_2$, so only the carbon attached to the ring remains as $\mathrm{COOH}$. The product is benzoic acid. Phenylacetic acid and acetophenone keep the second carbon.