Answer
a. the correct ionic formula is $AlCl_3$
b. the correct ionic formula is $CaS$
c.the correct ionic formula is $Li_2S$
d.the correct ionic formula is $Rb_3P$
e.the correct ionic formula is $CsI$
Work Step by Step
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. $ Al^{3+}$ and $Cl^-$. In this case to neutralize $Al^{3+}$ we need three $Cl^-$ anions
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
