Read the passage carefully and answer the questions.

Carboxylic acids are the organic compounds which comprise of carboxyl functional group, -COOH. These are generally found in nature, some higher members are known as fatty acids as they occur in natural fats as esters of glycerol. They can be prepared from oxidation of alcohols and aldehydes, hydrolysis of nitriles and amides and Grignard reagents. They are stronger acids than simple phenols and alcohols. but weaker than mineral acids. Substituents affect the acidity of aliphatic and aromatic carboxylic acids. They undergo reactions involving cleavage of -OH and C-OH bond. Carboxylic acids having an alpha hydrogen get halogenated in the presence of red phosphorus to form alpha halo carboxylic acids. The reaction is known as Hell Volhard Zelinsky reaction. Besides these aromatic carboxylic acids undergo electrophilic substitution reactions. They are versatile and are employed in the manufacture of various useful compounds. For example, methanoic acid is used in rubber, textile, dyeing, leather and electroplating industries. Higher fatty acids are used in the manufacture of soaps and detergents. Esters of Benzoic acid are used in perfumery.

The reagents needed for the preparation of Benzoic acid from ethyl benzene is

$KMnO_4-KOH, H_3O^+$

The correct answer is Option (2) → $KMnO_4-KOH, H_3O^+$

Core Concept

Any alkyl benzene having at least one benzylic hydrogen gets oxidized to benzoic acid on strong oxidation. The entire side chain ($-\text{CH}_2\text{CH}_3$ here) is converted into $-\text{COOH}$, irrespective of its length.

Strong oxidizing agents like alkaline $\text{KMnO}_4$ perform this side-chain oxidation.

Option-wise Explanation

1. $\text{AgNO}_3$ / $\text{H}_2\text{SO}_4$: This combination is not used for oxidation of side chains in alkyl benzenes. Silver nitrate in acid is generally used in qualitative tests (like halides), not for converting ethylbenzene to benzoic acid.
2. $\text{KMnO}_4$ - $\text{KOH}, \text{H}_3\text{O}^+$: Alkaline potassium permanganate is a powerful oxidizing agent. It oxidizes the benzylic carbon completely to a carboxyl group. Acid workup ($\text{H}_3\text{O}^+$) converts the benzoate salt into benzoic acid. This is the standard method.
3. $\text{H}_2$ / $\text{PtO}_2$: This is a reduction condition. It hydrogenates double bonds or reduces certain functional groups, but it does not oxidize an alkyl side chain to a carboxylic acid.
4. $\text{LiAlH}_4$ / Ether: $\text{LiAlH}_4$ is a strong reducing agent. It reduces carboxylic acids, esters, etc., to alcohols. It cannot oxidize ethylbenzene; instead, it performs the opposite type of reaction.