What is the spin-only magnetic moment value (B.M.) of a divalent metal ion with atomic number $25,$ in its aqueous solution?

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 :

Given below are two statements :

Statement I : The $\mathrm{E}°$ value of ${\mathrm{Ce}}^{4+}/{\mathrm{Ce}}^{3+}$ is $+1.74 \mathrm{V}$

Statement II : Ce is more stable in ${\mathrm{Ce}}^{4+}$ state than ${\mathrm{Ce}}^{3+}$ state.

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