The correct answer is option 4, Ca2+ and S2-. Isoelectronic ions are ions that have the same number of electrons. In this case, Ca2+ and S2- both have 18 electrons.

Option 1 is incorrect because Na+ and K+ have different numbers of electrons. Na+ has lost one electron and K+ has lost two electrons.

Option 2 is incorrect because K and Mg2+ have different numbers of electrons. K has 19 electrons while Mg2+ has 10 electrons.

Option 3 is incorrect because Mg2+ and Ca2+ have different numbers of electrons. Mg2+ has 10 electrons while Ca2+ has 18 electrons.

Therefore, the only correct pair of isoelectronic ions is Ca2+ and S2- because they both have the same number of electrons, which is 18.