The correct formula for an ionic compound that obeys the octet rule is: NaO, KBr2, GaCI, AIF3, CaO2. Why? 

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The formula that is correct as written is aluminum fluoride, AlF3.  Aluminum is in the boron group, group thirteen, which has three electrons in it's outer energy level.  Aluminum is also classified as a metal, meaning it donates it's electrons to satisfy the octet rule, rather than trying to find five more to add to it's outer energy level.  Fluorine is a member of the halogen family, group seventeen.  All the members of the halogens have room for one electron to complete their outer electron energy level.  The halogens are also very electronegative, meaning they willingly take donations from electron donors.  The three fluorine atoms willing accept aluminum's three electrons, producing an aluminum ion (+3) and three fluorine ions (-1, each).  The oppositely charged ions attract each other in a strong bond known as an ionic bond to produce aluminum fluoride.

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