What is the product X of the given reaction?

The correct answer is Option (1) → 3-(2-hydroxycyclohexyl)propanoic acid

The starting compound, 3-(2-oxocyclohexyl)propanoic acid, has two groups that could be reduced: a ketone C=O in the ring and a carboxyl group at the end of the side chain. Sodium borohydride is a mild reducing agent:

• Ketone group: reduced by \( \ce{NaBH4} \) to a secondary alcohol: \( \ce{>C=O} \xrightarrow{\ce{NaBH4}} \ce{>CH-OH} \).

• Carboxyl group: not reduced by \( \ce{NaBH4} \); it stays as –COOH.

So X keeps the three-carbon acid side chain and has an –OH on the ring carbon that carried the C=O: 3-(2-hydroxycyclohexyl)propanoic acid.

Why the other options are incorrect

Option (2) 3-(2-oxocyclohexyl)propanal (ring C=O kept, –COOH changed to –CHO) — Incorrect. It leaves the ketone untouched and changes the carboxyl group, the reverse of what \( \ce{NaBH4} \) does; \( \ce{NaBH4} \) does not reduce –COOH.

Option (3) 2-(2-hydroxyethyl)cyclohexan-1-ol (a diol) — Incorrect. Turning –COOH into an alcohol needs a strong reducing agent such as \( \ce{LiAlH4} \), not \( \ce{NaBH4} \); the side chain shown also has one carbon fewer than in the starting acid.

Option (4) Decalin (two fused rings, no functional group) — Incorrect. Reduction by \( \ce{NaBH4} \) changes a C=O group to CH–OH; it does not remove both functional groups and join the chain into a second ring.

NCERT (2026-27 edition):
8.4 Chemical Reactions – “2. Reduction
(i) Reduction to alcohols: Aldehydes and ketones are reduced to primary and secondary alcohols respectively by sodium borohydride (NaBH₄) or lithium aluminium hydride (LiAlH₄) as well as by catalytic hydrogenation (Unit 7, Class XII).”

8.9.3 Reactions Involving –COOH Group – “Carboxylic acids are reduced to primary alcohols by lithium aluminium hydride or better with diborane. Diborane does not easily reduce functional groups such as ester, nitro, halo, etc. Sodium borohydride does not reduce the carboxyl group.”