The electronic configurations of bivalent europium and trivalent cerium are

$[\mathrm{Xe}] 4 \mathrm{f}^{4}$ and $[\mathrm{Xe}] 4 \mathrm{f}^{9}$

[Xe] $4 \mathrm{f}^{7}$ and [Xe] $4 \mathrm{f}^{1}$

$[\mathrm{Xe}] 4 \mathrm{f}^{7} 6 \mathrm{~s}^{2}$ and $[\mathrm{Xe}] 4 \mathrm{f}^{2} 6 \mathrm{~s}^{2}$

$[\mathrm{Xe}] 4 \mathrm{f}^{2}$ and [Xe] $4 \mathrm{f}^{7}$