Which of the following elements belongs to the 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 one of the following is the correct order of ionic radii?

What is the general formula of lanthanoid hydroxide? (Consider $Ln$ as any lanthanoid element)

Arrange the following metal complexes/compounds in the increasing order of spin-only magnetic moment. Presume all three are high-spin systems.
(Atomic numbers: $Ce = 58$,$Gd = 64$,and $Eu = 63$.)
$(a)$ $(NH_4)_2[Ce(NO_3)_6]$
$(b)$ $Gd(NO_3)_3$
$(c)$ $Eu(NO_3)_3$
Answer is:

Which of the following ions is the strongest oxidizing agent? [ Atomic Number of $Ce = 58, Eu = 63, Tb = 65, Lu = 71$ ]