This is a nucleophilic substitution reaction where cyanide ion (CN⁻) from aqueous KCN acts as a nucleophile and attacks the carbon bonded to chlorine in 1-chloro-3-methylbutane. The chlorine is a good leaving group. Since the alkyl halide is primary, the reaction proceeds via an S${\mathrm{N}}_2$ mechanism, leading to the substitution product, 4-methylpentanenitrile.

The product is: $NC-\left({\mathrm{CH}}_2\right){)}_{2}CH\left({\mathrm{CH}}_3\right){)}_2$

Final Answer: The third option (structure with formula C₆H₁₁N)