Which of the following electrolyte can coagulate a given amount of ${\mathrm{As}}_2{\mathrm{S}}_3$ solution in minimum concentration.

• Hardy schulze rule: The ability of the flocculating ion to produce precipitation increases with valency          

Among the given complexes, the valency is in the order

      ${\mathrm{Al}}^{+3}>{\mathrm{Mg}}^{+2}>{\mathrm{K}}^{+}$ 

${\mathrm{As}}_2{\mathrm{S}}_3$ is a negatively charged sol and positively charged ions will cause it to coagulate 

• Aluminum has the highest charge among the other cations in aluminum nitrate, measuring +3. Therefore, it has the greatest ability to coagulate substances, and only a small amount of it is required to coagulate the specified sol. 

Hence the correct option is (B).