How many elements are there in the $4f$ series?

Lanthanoids $(Ln)$ are well known for their $+3$ oxidation state. Which of the following statements is $NOT$ applicable to them?

Identify the correct statements from the following :
$I$. The ionic radius of $Pr^{3+}, Dy^{3+}$ and $Sm^{3+}$ follow the order,$Sm^{3+} > Pr^{3+} > Dy^{3+}$.
$II$. $Eu^{2+}$ acts as a strong reducing agent.
$III$. $Pu$ exhibits $+7$ oxidation state.

The lanthanoid that does $NOT$ show $+4$ oxidation state is

The elements in which electrons are progressively filled in $4f$ orbital are called

Write a note on oxidation states shown by lanthanoids.