Consider the following statements.

$I. La{\left(OH\right)}_3$ is the least basic among hydroxides of lanthanides.

$II.Z{r}^{4+}andH{f}^{4+}$ possess almost the same ionic radii.

$III.C{e}^{4+}$ can act as an oxidizing agent. Which of the above is/are true?

i) The basic character of hydroxides decreases from $La{\left(OH\right)}_3$ to $Lu{\left(OH\right)}_3.$ Due to smaller size of $Lu,$ the $Lu-OHB6$ bond attains more covalent character.  Thus, the statement-I is incorrect.
ii) Due to lanthanide contraction ionic radius of $z{r}^{4+}$ and $H{f}^{4+}$ are nearly same, thus statement-II is true.
iii) The maximum oxidation state of $Ce$ is +4 it can be easily converted into lower oxidation state.
Thus $C{e}^{4+}$ can act as oxidizing agent.