1. Write the ground-state configurations of neutral Ce and Eu

   • Cerium, $Z=58$: $\mathrm{Ce} = [\mathrm{Xe}]\,4f^1 5d^1 6s^2$
   • Europium, $Z=63$: $\mathrm{Eu} = [\mathrm{Xe}]\,4f^7 6s^2$

2. Find the configuration of $\mathrm{Eu}^{2+}$

   Europium loses two electrons from the outermost shell first, i.e. from $6s$.

   $\mathrm{Eu}^{2+} : [\mathrm{Xe}]\,4f^7$

   This is especially stable because $4f^7$ is a half-filled configuration.

3. Find the configuration of $\mathrm{Ce}^{3+}$

   Start from: $\mathrm{Ce} = [\mathrm{Xe}]\,4f^1 5d^1 6s^2$

   On ionization, electrons are removed first from $6s$, then $5d$.

   Removing 3 electrons:

   • remove $2$ electrons from $6s$
   • remove $1$ electron from $5d$

   So, $\mathrm{Ce}^{3+} : [\mathrm{Xe}]\,4f^1$

4. Match with the options

   We need:

   • $\mathrm{Eu}^{2+} = [\mathrm{Xe}]\,4f^7$
   • $\mathrm{Ce}^{3+} = [\mathrm{Xe}]\,4f^1$

   This corresponds to Option D.

5. Check all options briefly

   • A: $[\mathrm{Xe}]4f^7 6s^2$ and $[\mathrm{Xe}]4f^2 6s^2$ → incorrect, ions should not retain $6s^2$ here.
   • B: $[\mathrm{Xe}]4f^2$ and $[\mathrm{Xe}]4f^7$ → incorrect.
   • C: $[\mathrm{Xe}]4f^4$ and $[\mathrm{Xe}]4f^9$ → incorrect.
   • D: $[\mathrm{Xe}]4f^7$ and $[\mathrm{Xe}]4f^1$ → correct.

Final Answer: $\boxed{\text{D}}$