To avoid the precipitation of hydroxides of ${\mathrm{Ni}}^{2+},{\mathrm{Co}}^{2+},{\mathrm{Mn}}^{2+}$ along with those of the third group cations, the solutions should be: 

The purpose of the strong electrolyte  ${\mathrm{NH}}_4\mathrm{Cl}$ is to prevent the ionisation of  ${\mathrm{NH}}_4\mathrm{OH}$, lowering the  concentration of OH ions in the solution. However, this is enough to precipitate the basic radicals of the  third group because group III hydroxides have a lower solubility product than group IV, V, and Vi  hydroxides.

Hence, the option B is correct.