The ion with the largest ionic radius of the options given is S^2 - (sulphide ion).
In fact, in a series of chemical elements with the same number of electrons, the ion with the lowest nuclear charge (fewer protons) will have the largest radius.
Here is a brief description:
- S^2- has 16 protons.
- Cl^1- has 17 protons.
- K^1+ has 19 protons.
- Ca^2+ has 20 protons.
Because S^2- has the fewest protons, its electrons have the least attraction to the nucleus, resulting in the largest ionic radius.
Regards from Italy,
Nitrotoluene