What is the major product of the following reaction?

The correct answer is Option (2) →

Alkyl side chains on benzene containing at least one benzylic hydrogen undergo oxidation with hot alkaline $KMnO_4$ to give benzoic acid, irrespective of chain length.

The given compound is propyl benzene:

$C_{6}H_{5}—CH_{2}CH_{2}CH_{3}$

Benzylic carbon ($—CH_{2}$ attached to ring) contains hydrogen.

Hot $KMnO_{4}$ oxidizes entire side chain to $—COOH$.

Thus:

$C_{6}H_{5}—CH_{2}CH_{2}CH_{3} \rightarrow C_{6}H_{5}—COOH$

Option 1: $C_{6}H_{5}—CH_{2}CH_{2}COOH$

Side chain is not partially oxidized.

Option 2: $C_{6}H_{5}—COOH$

Complete oxidation occurs $\rightarrow$ benzoic acid.

Option 3: $C_{6}H_{5}—CH_{2}COOH$

Incomplete oxidation does not occur.

Option 4: $\text{C}_6\text{H}_5\text{—}\text{CH}_2\text{COCH}_3$

Oxidation gives acid, not ketone.