The correct order of ionic radii of ${\mathrm{Y}}^{3+},{\mathrm{La}}^{3+},{\mathrm{Eu}}^{3+}$ and ${\mathrm{Lu}}^{3+}$ is :

The correct sequence is 

${\mathrm{Y}}^{3+}<{\mathrm{Lu}}^{3+}<{\mathrm{Eu}}^{3+}<{\mathrm{La}}^{3+}$. Because Eu and Lu are lanthanide series members, and La is a lanthanide series member, and ${\mathrm{Y}}^{3+}$When compared to La, ion has lower radii. ${\mathrm{La}}^{3+}$ because it is directly above ${\mathrm{La}}^{3+}$in the periodic table.