Q.On acid hydrolysis, propanenitrile gives _______.

DIBAL-H (di-isobutyl aluminium hydride) at low temperature reduces a nitrile only as far as the imine (which hydrolyses in workup to the aldehyde), stopping short of the amine — this is exactly the 'selective reducing agent' behaviour the reason statement describes, and it correctly explains the assertion.

Hex-4-ennitrile ($CH_3-CH=CH-CH_2-CH_2-C\equiv N$) treated with DIBAL-H (1 equivalent, usually at low temperature such as $-78^\circ C$) undergoes partial reduction of the nitrile's triple bond to a metalated imine intermediate; this is stable to further reduction under the reaction conditions because DIBAL-H, being bulky (two isobutyl groups) and used in controlled stoichiometry/temperature, does not reduce further. On aqueous acidic hydrolysis (workup), the imine hydrolyses to the corresponding aldehyde, hex-4-enal ($CH_3-CH=CH-CH_2-CH_2-CHO$) — so the assertion is a true, standard named transformation. The reason — that DIBAL-H is a selective reducing agent — is also true, and is precisely why the reaction stops cle …

Nitrile + $H_2O/H^+$ (complete hydrolysis) $\rightarrow$ carboxylic acid.

Propanenitrile, $CH_3CH_2CN$, on acid hydrolysis (dilute acid, heating under reflux) is first converted to propionamide ($CH_3CH_2CONH_2$) via partial hydrolysis, and on further/complete hydrolysis this amide is hydrolysed to the carboxylic acid, propanoic acid, releasing ammonium ion: …