Taking the Bohr radius as a₀ =53 pm, the radius of Li⁺⁺ ion in its ground state, on the basis of Bohr's model, will be about [NCERT Exemplar]

The atomic number of lithium is 3, therefore the radius of Li⁺⁺ ion in its ground state, on the basis of Bohr's model, will be about $\frac{1}{3}$ times to  that of Bohr radius.
Therefore, the radius of lithium ion is near $\frac{53}{3}$ =18 pm.