Which one of the following elements shows the maximum number of different oxidation states in its compounds?

What is the electronic configuration of $Nd^{2+}$?

The reason for the greater range of oxidation states in actinoids is attributed to:

Name a member of the lanthanoid series which is well known to exhibit $+4$ oxidation state.

Identify the correct statements about lanthanoids:
$I$. $Ce^{4+}$ and $Tb^{4+}$ act as oxidising agents.
$II$. $Eu^{2+}$ and $Yb^{2+}$ act as reducing agents.
$III$. Mischmetal is an alloy of $95 \%$ lanthanoid metal and $5 \%$ iron.
$IV$. $La^{3+}$ and $Ce^{4+}$ are diamagnetic in nature.

The correct order of ionic radii of $Ce$,$La$,$Pm$ and $Yb$ in $+3$ oxidation state is