Chemistry - f-block Question with Solution | TestHub

Lanthanide ion with no unpaired electron is diamagnetic in nature.

Lanthanoid is also called lanthanide, which can be defined as any of the series of $15$ consecutive chemical elements in the periodic table from lanthanum to lutetium (atomic numbers $57-71$). With the elements scandium and yttrium, they make up the rare-earth metals. Lanthanides are also radioactive in nature.

$\mathrm{Ce}:\left[\mathrm{Xe}\right]4{\mathrm{f}}^{1}5 {\mathrm{d}}^{1}6 {\mathrm{s}}^2;{\mathrm{Ce}}^{4+}$ diamagnetic

$\mathrm{La}:\left[\mathrm{Xe}\right]4{\mathrm{f}}^{0}5 {\mathrm{d}}^{1}6 {\mathrm{s}}^2;{\mathrm{La}}^{3+}$ diamagnetic

Hence, the answer is option B.