(C) The basic strength of lanthanoid hydroxides,$Ln(OH)_3$,decreases as the ionic radius of the $Ln^{3+}$ ion decreases due to lanthanoid contraction.

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(C) The basic strength of lanthanoid hydroxides,$Ln(OH)_3$,decreases as the ionic radius of the $Ln^{3+}$ ion decreases due to lanthanoid contraction.As we move from $La$ to $Lu$ in the lanthanoid series,the ionic radius decreases.Therefore,$La(OH)_3$ is the most basic,and $Lu(OH)_3$ is the least basic (weakest base).