The electronic configurations of Eu (Atomic No. 63), Gd (Atomic No. 64) and Tb (Atomic No. 65) are

• A

  $\left[\mathrm{Xe}\right]4{\mathrm{f}}^{6}5{\mathrm{d}}^{1}6{\mathrm{s}}^2,\left[\mathrm{Xe}\right]4{\mathrm{f}}^{7}5{\mathrm{d}}^{1}6{\mathrm{s}}^2\text{ and }\left[\mathrm{Xe}\right]4{\mathrm{f}}^{8}5{\mathrm{d}}^{1}6{\mathrm{s}}^2$

• B

  $\left[\mathrm{Xe}\right]4{\mathrm{f}}^{7}6{\mathrm{s}}^2,\left[\mathrm{Xe}\right]4{\mathrm{f}}^{7}5{\mathrm{d}}^{1}6{\mathrm{s}}^2\text{ and }\left[\mathrm{Xe}\right]4{\mathrm{f}}^{9}6{\mathrm{s}}^2$

• C

  $\left[\mathrm{Xe}\right]4{\mathrm{f}}^{7}6{\mathrm{s}}^2,\left[\mathrm{Xe}\right]4{\mathrm{f}}^{8}6{\mathrm{s}}^2\text{ and }\left[\mathrm{Xe}\right]4{\mathrm{f}}^{8}5{\mathrm{d}}^{1}6{\mathrm{s}}^2$

• D

  $\left[\mathrm{Xe}\right]4{\mathrm{f}}^{6}5{\mathrm{d}}^{1}6{\mathrm{s}}^2,\left[\mathrm{Xe}\right]4{\mathrm{f}}^{7}5{\mathrm{d}}^{1}6{\mathrm{s}}^2\text{ and }\left[\mathrm{Xe}\right]4{\mathrm{f}}^{9}6{\mathrm{s}}^2$