In this question, we are asked to identify the pair of ions that are isoelectronic, which means they have the same number of electrons. A neutral atom of sodium (Na) has 11 electrons. When it loses one electron, it forms a sodium ion (Na+) with 10 electrons. On the other hand, a neutral atom of oxygen (O) has 8 electrons. When it gains two electrons, it forms an oxygen ion (O2-) with 10 electrons. Therefore, the sodium ion and the oxygen ion are isoelectronic since they both have 10 electrons.

Option 1, Mg2+ and Ar: A neutral atom of magnesium (Mg) has 12 electrons. When it loses two electrons, it forms a magnesium ion (Mg2+) with 10 electrons. Argon (Ar) is a noble gas with 18 electrons, so Mg2+ and Ar are not isoelectronic.

Option 3, Al3+ and Cl-: A neutral atom of aluminum (Al) has 13 electrons. When it loses three electrons, it forms an aluminum ion (Al3+) with 10 electrons. Chlorine (Cl) is a halogen with 17 electrons. Therefore, Al3