Which of the following lanthanoids shows $+4$ oxidation state to acquire noble gas configuration?
(At nos. : $La=57, Ce=58, Eu=63$ and $Yb=70$ )

The point of dissimilarity between lanthanides and actinides is

What are the elements of the $f$-block called?

Which actinoid from the following in its $+3$ state has the largest size?

Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R$:
Assertion $A$: Size of $Bk^{3+}$ ion is less than $Np^{3+}$ ion.
Reason $R$: The above is a consequence of the lanthanoid contraction.
In the light of the above statements,choose the correct answer from the options given below:

If $Ln$ represents a lanthanoid element,find the general formula of lanthanoid hydroxide.