CI- is a chloride ion and has 18 electrons. To be isoelectronic with CI-, an ion or atom must have the same number of electrons.

Option 1, K+, is a potassium ion and has 19 electrons. It has one more electron than CI-, so it is not isoelectronic with CI-.

Option 2, Mg2+, is a magnesium ion and has 10 electrons. It has eight fewer electrons than CI-, so it is not isoelectronic with CI-.

Option 3, S2-, is a sulfide ion and has 18 electrons. It has the same number of electrons as CI-, so it is isoelectronic with CI-.

Option 4, P3-, is a phosphide ion and has 15 electrons. It has three fewer electrons than CI-, so it is not isoelectronic with CI-.

Therefore, option 2 (Mg2+) is the correct answer since it is the only ion that is not isoelectronic with CI-.