Which of the following is the correct decreasing order of oxidizing power of perhalate ions?

Which of the following elements is doped to a fiber amplifier in an optical fiber communication system?

Which of the following comparisons is correct for $\Delta_0$ of the following complexes?
I- $[\text{Co(H}_2\text{O)}_6]^{2+}$
II- $[\text{Co(H}_2\text{O)}_6]^{3+}$
III- $[\text{Fe(H}_2\text{O)}_6]^{3+}$
IV- $[\text{Fe(CN)}_6]^{3+}$

Among the given ligands, the negative ligand is