Which of the following element has $f^7$ electronic configuration in its + 4 state

The electronic configuration of $\mathrm{Ac} \mathrm{is} \left[\mathrm{Rn}\right]6{\mathrm{d}}^{1}7{\mathrm{s}}^2$

The electronic configuration of $\mathrm{Bk} \mathrm{is} \left[\mathrm{Rn}\right]5{\mathrm{f}}^{9}7{\mathrm{s}}^2$

The electronic configuration of $\mathrm{Er} \mathrm{is} \left[\mathrm{Xe}\right]4{\mathrm{f}}^{12}6{\mathrm{s}}^2$

The electronic configuration of $\mathrm{Lr} \mathrm{is} \left[\mathrm{Rn}\right]5{\mathrm{f}}^{14}6{\mathrm{d}}^{1}7{\mathrm{s}}^2$

Hence, the correct answer is B, $\mathrm{Bk}$