Which element from the following forms colourless compounds in $+2$ oxidation state?

Which of the following may be colourless?

The increase in the atomic radii of the third $(5d)$ series of transition elements is very small, which may be accounted for by the filling of '$X$' orbitals before '$Y$' orbitals. $X$ and $Y$ are:

Which of the following forms a colourless solution in aqueous medium?

The type of orbitals present in $Fe$ is

For $Cr, Mn, Fe,$ and $Co$,the $E_{{M^{3+}}/{M^{2+}}}^{\circ}$ values are $0.41, +1.57, +0.77,$ and $+1.97 \ V$ respectively. Which of these metals is most easily oxidized from the $+2$ to the $+3$ oxidation state?