Step 1: Understanding the Concept
Phenyl ethene (styrene) is \(\text{C}_{6}\text{H}_{5}\text{CH}=\text{CH}_{2}\). Acidic \(\text{KMnO}_{4}\) cleaves the double bond.
Step 2: Oxidation
The ring side carbon is oxidised to \(-\text{COOH}\), and the terminal \(\text{CH}_{2}\) is oxidised to \(\text{CO}_{2}\) and water.
Step 3: Product
The ring keeps one \(-\text{COOH}\) group, giving \(\text{C}_{6}\text{H}_{5}\text{COOH}\), benzoic acid. This is option (C).
Final Answer:
The product is benzoic acid.
\[ \boxed{\text{(C) }\text{Benzoic acid}} \]