Which of the following oxidation states is the most common among the lanthanoids?

Which of the following lanthanoids exhibits a $+4$ oxidation state with an $f^7$ configuration?

Identify the correct decreasing order of ionic radii of the following lanthanoids: $Ce^{3+}$,$Pm^{3+}$,$Sm^{3+}$,and $Gd^{3+}$.

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 trivalent ion having the largest size in the lanthanide series is:

Which element from the following has the largest ionic size in the $+3$ state?