Which of the following will have the highest coagulating power for Fe(OH)₃. colloid?

• A

  ${\mathrm{PO}}_4^{3-}$

• B

  ${\mathrm{SO}}_4^{2-}$

• C

  ${\mathrm{Ca}}^{2+}$

• D

  ${\mathrm{Al}}^{3+}$