$\mathrm{Ce}$ is the first element of lanthanide series.

What Is the most suitable $\text{ E.C. }$of  ${\mathrm{Ce}}^{3+}$

The electronic configuration can be given as,

$\mathrm{Ce}:\left[\mathrm{Xe}\right]4{\mathrm{f}}^{1}5{\mathrm{d}}^{1}6{\mathrm{s}}^2$

Therefore, $\text{ [Xe] }4{\mathrm{f}}^1$ is the electronic configuration of the ${\mathrm{Ce}}^{3+}$ ion.