The conditions and consequence that favours the $t_{2 \mathrm{~g}}{ }^3 \mathrm{e}_{\mathrm{g}}{ }^1$ configuration in a metal complex are :

When Ethane-1,2-diamine is added progressively to an aqueous solution of Nickel (II) chloride, the sequence of colour change observed will be:

One mole of the octahedral complex compound $\mathrm{Co}\left(\mathrm{NH}_3\right)_5 \mathrm{Cl}_3$ gives 3 moles of ions on dissolution in water. One mole of the same complex reacts with excess of $\mathrm{AgNO}_3$ solution to yield two moles of $\mathrm{AgCl}_{(\mathrm{s})}$. The structure of the complex is:

Identify the coordination complexes in which the central metal ion has $\mathrm{d}^4$ configuration.

(A) $\left[\mathrm{FeO}_4\right]^{2-}$

(B) $\quad\left[\mathrm{Mn}(\mathrm{CN})_6\right]^{3-}$

(C) $\left[\mathrm{Fe}(\mathrm{CN})_6\right]^{3-}$

(D) (E) $\left[\mathrm{NiF}_6\right]^{2-}$

Choose the correct answer from the options given below :