The electronic configuration of actinoids cannot be assigned with degree of certainty because of

Given below are two statement : one is labelled as Assertion A and the other is labelled as Reason R :

Assertion A : Size of ${\mathrm{Bk}}^{3+}$ ion is less than ${\mathrm{Np}}^{3+}$ ion.

Reason R : The above is a consequence of the lanthanide contraction.

In the light of the above statements, choose the correct answer from the options given below :

Give reason for actinides show irregularities in their electronic configuration.