The stability of divalent lanthanide ions depends on their electronic configuration. Fully filled and half-filled configurations exhibit enhanced stability, as seen in: \[\text{Ce}^{2+} \to 4f^1, \quad \text{Sm}^{2+} \to 4f^6, \quad \text{Eu}^{2+} \to 4f^7, \quad \text{Yb}^{2+} \to 4f^{14}.\]
Reduction potential comparison: \[\
E^\circ_{M^{3+}/M^{2+}}: \quad \text{Eu} = -0.35 \, \text{V}, \quad \text{Yb} = -1.05 \, \text{V}.\]
Europium (\( \text{Eu}^{2+} \)) is more stable than \( \text{Yb}^{2+} \) due to its lower reduction potential and the greater stability of its half-filled \( 4f^7 \) configuration.
Final Answer: \[\boxed{\text{Europium} \, (\text{Eu}^{2+})}\]