Answer
Please see the work below.
Work Step by Step
We know that
a. Sodium ion has +1 charge while oxide has -2 charge so the correct formula is $Na_2O.$
b. Iron ion has +2 charge while sulfide has -2 charge so the correct formula is $FeS.$
c. CO ion has +3 charge while $SO_4$ has -2 charge so the correct formula is $CO_2(SO_4)_3.$
d. Barium ion has +2 charge while fluoride has 1 charge so the correct formula is $BaF_2.$
