Identify the product ' $B$ ' in the following sequence of reactions. Ethanenitrile $\xrightarrow{\mathrm{SnCl}_2, \mathrm{HCl}} \mathrm{A} \xrightarrow{\mathrm{H}_3, \mathrm{O}^{+}} \mathrm{B}+\mathrm{NH}_4 \mathrm{Cl}$

Ethanenitrile undergoes Stephen reduction with $\mathrm{SnCl}_2$ and $\mathrm{HCl}$ to form ethanimine hydrochloride as product A: $\mathrm{CH_3CN} + \mathrm{SnCl}_2 + 2\mathrm{HCl} \rightarrow \mathrm{CH_3CH=NH \cdot HCl}$.

Hydrolysis of A with $\mathrm{H_3O^+}$ yields ethanal and ammonium chloride: $\mathrm{CH_3CH=NH \cdot HCl} + \mathrm{H_2O} \xrightarrow{\mathrm{H_3O^+}} \mathrm{CH_3CHO} + \mathrm{NH_4Cl}$.

Product B is ethanal ($\mathrm{CH_3CHO}$), corresponding to option D.

$\boxed{\mathrm{D}}$