Among the following hydroxides, the most basic is

Because of lanthanoid contraction, the ionic radius of $\mathrm{Ln}{}^{3+}$ decreases from La to Lu, so the $\mathrm{Ln}-\mathrm{OH}$ bond gains covalent character and the basic strength of $\mathrm{Ln}(\mathrm{OH}){}_3$ falls. Hence $\mathrm{La}(\mathrm{OH}){}_3$ is the most basic and $\mathrm{Lu}(\mathrm{OH}){}_3$ the least.