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

Which of the following compounds is the most acidic?

Which of the following elements are included in the actinoid series?

In which of the following properties do $Lanthanoids$ and $Actinoids$ show similarity?

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:

The percentage of lanthanides and iron,respectively,in misch metal are