The correct option is C Z is CH3CH2CH2CH2COOH
Reaction 1: Sodium borohydride is a moderate reducing agent. It is very effective for reduction of aldehyde and ketones to alcohols (X). By itself, it will not generally reduce esters, carboxylic acids, or amides (although it will reduce acyl chlorides to alcohols).
Reaction 2: Primary alcohol react with PCl5 to produce alkyl chloride (Y).
Reaction 3: In the second step, the −Cl atom of alkyl chloride is displaced with cyano group by treatment with KCN. in the last step, cyano group is hydrolyzed to carboxylic group in acidic medium (Z).
CH3CH2CH2CHONaBH4−−−−→CH3CH2CH2CH2OHPCl5−−−→CH3CH2CH2CH2Cl(i)KCN−−−−−→(ii)H+CH3CH2CH2CH2COOH X ‘ Y Z
So, the product Z is CH3CH2CH2CH2COOH
Hence, option "a" is the correct answer.