Final answer:
K+ and Ca²+ ions are isoelectronic with Ar, as both ions have the same number of electrons as a neutral Argon atom, which is 18.
Step-by-step explanation:
The ions that are isoelectronic with Argon (Ar) are those that have the same number of electrons as Argon, which has a total of 18 electrons in its neutral state. The ions K+ (potassium ion) with 18 electrons and Ca²+ (calcium ion) with 18 electrons are both isoelectronic with Argon because when potassium loses one electron, it has 18 electrons like Argon, and similarly, when calcium loses two electrons, it has 18 electrons like Argon.