On which factors does the stability of an oxidation state in Lanthanoid elements depend?

The most common lanthanide is

Transuranium elements are

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:

Which of the following statements concerning lanthanide elements is false?

Which one amongst the following are good oxidizing agents?
$A.$ $Sm^{2+}$
$B.$ $Ce^{2+}$
$C.$ $Ce^{4+}$
$D.$ $Tb^{4+}$
Choose the most appropriate answer from the options given below: