Styrene is also known as phenyl ethene. It can be converted into benzoic acid by reacting with either KMnO4 or with dilute H2SO4 in the presence of heat. The end product of this reaction would be benzoic acid and carbon dioxide.
Styrene (Phenyl ethene) is treated with potassium permanganate, an aqueous solution of potassium hydroxide which gives an intermediate potassium benzoate. Then this potassium benzoate is treated with H3O+ which gives benzoic acid.