Treatment of an alkylbenzene with potassium permanganate results in oxidation to give benzoic acid. The position directly adjacent to an aromatic group is called the “benzylic” position. The reaction only works if there is a hydrogen attached to the carbon. Hence, toluene, on oxidation with potassium permanganate, gives benzoic acid $\left(\mathrm{A}\right)$.

Carboxylic acids reacts with thionyl chloride to form acid chlorides. Hence, benzoic acid, on reaction with thionyl chloride $\left({\mathrm{SOCl}}_2\right)$, gives benzoyl chloride $\left(\mathrm{B}\right)$.

Next reaction is the Rosenmund reaction. It is a reaction where acid chlorides are converted into aldehydes by using hydrogen gas over palladium poisoned by barium sulfate. Hence, benzoyl chloride upon reduction with ${\mathrm{H}}_2/\mathrm{Pd}$ or ${\mathrm{BaSO}}_4$, forms benzaldehyde $\left(\mathrm{C}\right)$.