Answer
Choice (d) S$^{2-}$ $\gt$ Cl$^{-}$ $\gt$ K$^{+}$ is the correct answer
Work Step by Step
All of the following ions are isoelectronic meaning that they have the same number of electrons but have different numbers of protons. Therefore due to the attractive properties in an atom if the electrons remain constant then the more protons it posses results in a smaller atom. This is because the more protons an atom has the higher the attractive force the nucleus will have with the surrounding electrons which will pull the electrons closer to the nucleus making the overall atom radius smaller. Thus, because K most protons followed by Cl and then S the size comparison of their respective ions yields: S$^{2-}$ $\gt$ Cl$^{-}$ $\gt$ K$^{+}$. Therefore Choice (d) is the correct answer.
