Which of the following is most effective in coagulating a ferric hydroxide sol?

The positive charge of ferric hydroxide sol is due to the colloidal particles in ferric hydroxide.
According to the Hardy-Schulze rule, the active ion that causes the ferric hydroxide solution to coagulate must be negatively charged, with higher negative charges resulting in more effective coagulation.

This coordination species, along with ${\mathrm{K}}_3\left[\mathrm{Fe}(\mathrm{CN}{)}_6\right]\to 3{\mathrm{K}}^{+}+{\left[\mathrm{Fe}(\mathrm{CN}{)}_6\right]}^{3-}$, has the largest negative charge.

Therefore, the best coagulating active ion is option D.