The given Benzene derivative contains an ethyl group and a tertiary butyl group. As we know when Alkylbenzene is treated with KMnO₄ it results in the oxidation of alkyl group and forms Benzoic Acid.
In our case benzene has two alkyl groups. The carbon next to benzene carbon is called benzylic carbon. If this position has any H atom then it will be oxidized, otherwise the groups like tert. Butyl having no benzylic hydrogen are not oxidized.
The reaction is as below,
