Which element from the following has a completely filled $f$-orbital in both observed and expected electronic configurations?

Which lanthanoid element has the electronic configuration $[Xe]4f^7 5d^1 6s^2$?

Which of the following $f$-block elements exhibits the highest oxidation state?

The main reason for the larger number of oxidation states exhibited by the actinoids compared to the corresponding lanthanoids is:

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.

The lanthanide for which $+II$ and $+III$ oxidation states are common is: