Which one of the following exhibits the largest number of oxidation states?

The placement of $Zn,$ $Cd,$ and $Hg$ along with $d-$block elements is not considered proper because:

The transition elements have a characteristic electronic configuration which can be represented as:

Match the properties given in List-$I$ with the metals given in List-$II$:

List-$I$ (Property)	List-$II$ (Metal)
$(A)$ Element that can show $+8$ oxidation state	$(1)$ $Mn$
$(B)$ $3d$ series element that can show up to $+7$ oxidation state	$(2)$ $Cr$
$(C)$ $3d$ series element with maximum melting point	$(3)$ $Os$
	$(4)$ $Fe$

Why are the $E^o$ values for $Mn^{2+}/Mn$,$Zn^{2+}/Zn$,and $Ni^{2+}/Ni$ more negative than expected? The table below provides the values:
| $E^o_{Mn^{2+}/Mn}$ | $E^o_{Zn^{2+}/Zn}$ | $E^o_{Ni^{2+}/Ni}$ |
| :--- | :--- | :--- |
| $-1.18 \ V$ | $-0.76 \ V$ | $-0.25 \ V$ |

The outer electronic configuration of $Gd \ (At. \ No. \ 64)$ is