Identify the factor responsible for the greater range of oxidation states in actinoids.

Identify the lanthanoid element from the following.

What is the highest oxidation state exhibited by actinoids?

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.

Which of the following factors may be regarded as the main cause of lanthanide contraction?

What is the correct electronic configuration of $Th$ $(Z=90)$?