${\mathrm{CH}}_3-{\mathrm{CH}}_2-\mathrm{CN}\underset{\mathrm{Ether}}{\overset{{\mathrm{CH}}_3\mathrm{MgBr}}{\to }}\mathrm{A}\stackrel{{\mathrm{H}}_3{\mathrm{O}}^{+}}{\to }\mathrm{B}\underset{\mathrm{HCl}}{\overset{\mathrm{Zn}-\mathrm{Hg}}{\to }}\mathrm{C}$. The correct structure of $\mathrm{C}$ is

First, in this reaction the nitrile will react with the Grignard reagent. The reaction can be written as:

In this step, $\mathrm{Mg} \mathrm{Br}$ gets attached to the nitrogen atom and alkyl anion attacks on the electrophilic carbon of the nitrile.This step leads to the formation of  imine.

Now, this reaction will further take place accordingly in the presence of aqueous acid. The reaction can be written as:

Clemmenson reagent acts only on aldehydes and ketones. Therefore, butan-$2$-one will get reduced to butane when we treat it with zinc and hydrochloric acid.