Benzene is treated with (CH3)2CHCH2Cl and anhydrous AlCl3, and the major product is then heated with hot alkaline KMnO4 followed by acidification.…

The primary carbocation from isobutyl chloride undergoes a 1,2-hydride shift to the more stable tertiary carbocation $\mathrm{(CH_3)_3C^+}$, so Friedel-Crafts gives tert-butylbenzene, not isobutylbenzene. Side-chain oxidation by $\mathrm{KMnO_4}$ requires at least one benzylic hydrogen; the benzylic carbon of the tert-butyl group has none, so the side chain survives and the ring is not attacked. The final product is unchanged tert-butylbenzene.