The Lanthanoids

Due to lanthanoid contraction, the basic character of hydroxides decreases from to

As a result, covalent nature of bond also increases. Thus, the most basic hydroxide among the given option is

The stability of the divalent state in lanthanide ions is influenced by their electronic configuration. Fully filled and half-filled configurations are particularly stable:
Reduction potential comparison:
Europium ( ) is more stable than , as it has a lower reduction potential and a more stable half-filled configuration. Final Answer:

Across the lanthanide series, as we move from La to Lu , electrons are added to the 4f orbitals. These electrons do not shield each other effectively due to the poor shielding effect of f-orbitals. As a result, the effective nuclear charge increases across the series, pulling the electrons closer to the nucleus. This leads to a gradual decrease in the ionic radii—a phenomenon known as lanthanide contraction.