Butanenitrile is formed by the reaction of KCN with

Given data : It is given to identify  Butanenitrile from a compound by the reaction of $KCN$.

Explanation:

Propyl chloride is supposed to be heated with $KCN$ to prepare butanenitrile.

The chemical reaction is as follows. 
$C{H}_3-C{H}_2-C{H}_2-Cl\underset{KCN}{\overset{S_{N}2}{\to }}C{H}_3-C{H}_2-C{H}_2-CN$
In the above chemical reaction, the chloride ion is replaced by a cyanide ion through $S_{N}2$ reaction.

Hence, the correct aoption (D).Propyl chloride