On which factors does the stability of an oxidation state in Lanthanoid elements depend?

On the basis of Lanthanoid contraction,explain the following :
$(i)$ Nature of bonding in $La_2O_3$ and $Lu_2O_3$.
$(ii)$ Trends in the stability of oxo salts of lanthanoids from $La$ to $Lu$.
$(iii)$ Stability of the complexes of lanthanoids.
$(iv)$ Radii of $4d$ and $5d$-block elements.
$(v)$ Trends in acidic character of lanthanoid oxides.

The ion with $4f^7$ configuration is

Most common oxidation states shown by cerium are

The percentage of lanthanides and iron,respectively,in misch metal are

The electronic configurations of $Eu$ (Atomic No. $63$),$Gd$ (Atomic No. $64$) and $Tb$ (Atomic No. $65$) are