Answer
a. the correct symbols are $K^+$ and $S^{2-}$ the ionic formula is $K_2S$
b.the correct symbols are $Na^+$ and $N^{3-}$ the ionic formula is $Na_3N$
c.the correct symbols are $Al^{3+}$ and $I^-$ the ionic formula is $AlI_3$
d.the correct symbols are $Ga^{3+}$ and $O^{2-}$ the ionic formula is $Ga_2O_3$
Work Step by Step
using the Periodic Table 6.2 on page 187 we will find which atoms lose valence electrons and which ones gain valence atoms. Also, it shows the amount of electrons lost in groups 1A,2A,3A and gained in groups 5A,6A,7A
for example: atoms from group 1A lose 1 valence electron, turning into a cation with positive charge ( $Na^+$). Atoms from group 7A gain 1 valence electron, turning into an anion with negative charge ( $F^-$)
a. potassium belongs to group 1A ( lose 1 electron) $K^+$ and sulfur belong to 6A ( gain 2 electrons) $S^{2-}$
repeat with the rest of the questions.
The ionic formula unit represents the proportion expressed by the lowest number of cations and anions that will neutralize the compound's charge. So, the number of electrons given by one atom must be the same as the number of electrons received by other.
a. $K^+$ and $S^{2-}$. In this case, to neutralize $S^{2-}$ − we need two $K^+$ cations ($K_2S$)
repeat that with the rest of the compounds, basically changing the charge of the anion with the subscript of the cation and the cation's charge with the anion's subscript.
Remember, the cation is always written before the anion.
you can study this at pages 189-191 topic 6.2 - writing formulas for ionic compounds
