Which among the following lanthanoids forms the most stable $+2$ 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 elements in which electrons are progressively filled in $4f$ orbital are called

The number of incomplete orbitals in inner transition elements is

Which of the following statements concerning lanthanide elements is false?

Arrange the ions $Ce^{3+}$,$La^{3+}$,$Pm^{3+}$,and $Yb^{3+}$ in the increasing order of their ionic radii.