Paragraph: An aqueous solution of metal ion $\mathbf{M} 1$ reacts separately with reagents $\mathbf{Q}$ and $\mathbf{R}$ in excess to give tetrahedral and square planar complexes, respectively. An aqueous solution of another metal ion M2 always forms tetrahedral complexes with these reagents. Aqueous solution of $\mathbf{M} 2$ on reaction with reagent $\mathbf{S}$ gives white precipitate which dissolves in excess of $\mathbf{S}$. The reactions are summarized in the scheme given below:

Question: Reagent $\mathbf{S}$ is

• A

  ${\mathrm{K}}_4\left[\mathrm{F}\mathrm{e}{\left(\mathrm{C}\mathrm{N}\right)}_6\right]$
• B

  $\mathrm{N}{\mathrm{a}}_2\mathrm{H}\mathrm{P}{\mathrm{O}}_4$
• C

  ${\mathrm{K}}_2\mathrm{C}\mathrm{r}{\mathrm{O}}_4$
• Correct answer
  D

  $\mathrm{KOH}$

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