The electronic configurations of $\text{Eu}$ (Atomic no. $63$), $\text{Gd}$ (Atomic no. $64$) and $\text{Tb}$ (Atomic no. $65$) are:

1. A
   $[\text{Xe}]4f^{6}5d^{1}6s^2,[\text{Xe}]4f^{7}5d^{1}6s^2\text{ and }[\text{Xe}]4f^{9}6s^2$
2. B
   $[\text{Xe}]4f^{6}5d^{1}6s^2,[\text{Xe}]4f^{7}5d^{1}6s^2\text{ and }[\text{Xe}]4f^{8}5d^{1}6s^2$
3. C
   $[\text{Xe}]4f^{7}6s^2,[\text{Xe}]4f^{7}5d^{1}6s^2\text{ and }[\text{Xe}]4f^{9}6s^2$
4. D
   $[\text{Xe}]4f^{7}6s^2,[\text{Xe}]4f^{8}6s^2\text{ and }[\text{Xe}]4f^{8}5d^{1}6s^2$