Ionic potential is maximum in case of

The ionic potential is the ratio of the electrical charge (z) to the radius (r) of an ion. This ratio is a measure of the charge density at the surface of the ion, usually the denser the charge, the stronger the bond formed by the ion with ions of opposite charge.
Out of the given options, Lithium has the highest ionic potential value. On moving down the group, the atomic number of alkali metals increases and their ionization potential value decreases.