Butanitrile is formed by reaction of KCN with

Propyl chloride

The reaction of propyl chloride with KCN is a nucleophilic substitution reaction, in which the cyanide ion \((CN^-)\) attacks the propyl chloride molecule and displaces the chloride ion.  The reaction proceeds through an \(S_N2\) mechanism, which means that the nucleophile \((CN^-)\) attacks the electrophile (propyl chloride) from the back, resulting in inversion of stereochemistry.

Therefore the correct answer is (2).