Concept:
Strong oxidizing agents like $\mathrm{KMnO_4}$ oxidize any alkyl side chain attached to a benzene ring to a carboxylic acid group, provided there is at least one benzylic hydrogen.
Step 1: Identify substrate.
n-propylbenzene has a side chain with benzylic hydrogen.
Step 2: Oxidation effect.
Entire side chain is oxidized to $\mathrm{-COOH}$ regardless of length:
\[
\mathrm{C_6H_5CH_2CH_2CH_3 \xrightarrow{KMnO_4/KOH,\ \Delta} C_6H_5COOH}
\]
Step 3: Product formed.
\[
\text{Benzoic acid}
\]