Which among the following lanthanoids shows only $+3$ oxidation state?

In lanthanides,with increase in atomic number,atomic radius decreases,except for the element $\underline{ X }$. What is $\underline{ X }$?

What is the correct order of basic strength of the following metallic hydroxides?

Identify all the correct statements for Lanthanide contraction.
$(A)$ The covalent properties of the Lanthanide metal hydroxides increase from $La$ to $Lu$.
$(B)$ The chemical reactivity decreases from $La$ to $Lu$.
$(C)$ $La(OH)_3$ is more basic than $Lu(OH)_3$.
$(D)$ $Zr$ and $Hf$ have about the same radius.
$(E)$ Separation of Lanthanides from one another is easy.

Which of the following statements are correct?
$I$. $P$ has the least negative electron gain enthalpy among $P, S, Cl$, and $F$.
$II$. In $Eu$ and $Yb$, lanthanoid contraction is not observed.
$III$. $Ce(OH)_3$ is the most basic among lanthanoid hydroxides.
$IV$. The radii of $Na$ and $Na^+$ are $95 \ pm$ and $186 \ pm$ respectively.

Which of the following elements belongs to the actinoid series?