For the coagulations of positively charged hydrated ferric - oxide sol, the flocculating power of the ions is in the order:

• A

  $\mathrm{PO}_4^{3-}\gt\mathrm{SO}_4^{2-}\gt\mathrm{Cl}^{-}\gt\left[\mathrm{Fe}(\mathrm{CN})_6\right]^{4-}$
• B

  $\mathrm{Cl}^{-}\gt\mathrm{SO}_4^{2-}\gt\mathrm{PO}_4^{3-}\gt\left[\mathrm{Fe}(\mathrm{CN})_6\right]^{4-}$
• C

  $\mathrm{SO}_4^{2-}=\mathrm{Cl}^{-}=\mathrm{PO}_4^{3-}=\left[\mathrm{Fe}(\mathrm{CN})_6\right]^{4-}$
• Correct answer
  D

  $\left[\mathrm{Fe}(\mathrm{CN})_6\right]^{4-}\gt\mathrm{PO}_4^{3-}\gt\mathrm{SO}_4^{2-}\gt\mathrm{Cl}^{-}$

Hence, the correct answer is option: 'D'.