Which of the following element in +1 oxidation state has largest ionic radius?

The ionic radius of alkali metals increases down the group. The number of occupied shells in $\mathrm{Rb}^{+}$ (4 shells) is more than that in $\mathrm{K}^{+}$(3 shells), $\mathrm{Na}^{+}$(2 shells) and $\mathrm{Li}^{+}$(1 shell). Thus, it experiences smaller effective nuclear charge. $\therefore \quad \mathrm{Rb}^{+}$has largest ionic radius than among other elements.