Mg is not precipitated in V group because :

Magnesium does not precipitate in V group because the value of solubility product of magnesium carbonate is high.

${\mathrm{MgCO}}_3\rightleftharpoons {\mathrm{Mg}}^{+2}+{\mathrm{CO}}_3^{-2}$

Although there are relatively few ${\mathrm{CO}}_3^{2-}$ ions present, this does not stop the ionic product of  ${\mathrm{Mg}}^{2+}$ and ${\mathrm{CO}}_3^{2-}$ ion from decreasing the solubility product when ${\mathrm{NH}}_4\mathrm{Cl}$ is present.

Hence, correct option is B.