The list of the compounds is missing.
I will show you how to do it with a list of an exercise with similar statement.
This is the list of the ionic compounds to work:
a) NiO
b) MnO2
c) Cr2O3
d) MoO3
Furthermore, I trust this list is the same or very similar to your list, but the important thing is you will learn the procedure.
1) Identify the oxidation state (which is the same as the charge) of the element or elements that can only work with a definite charge.
2) In this case, given that all the products are metallic oxydes, the oxygen has oxidation stated 2-.
3) Given that the compound is neutral, the sum of the charges of the anion equals the charge of the metallic element with opposite side.
a) NiO: x + (-2) = 0 => x - 2 = 0 => x = 2 = 2+.
Answer the charge of the metallic element, Ni, is 2+
b) MnO2: x + 2*(2-) = 0 => x - 4 = 0 => x = 4+
Answer: the charge of the metallic Mn is 4+.
c) Cr2O3: 2x + 3(2-) = 0 => 2x - 6 = 0 => 2x = 6 => x = 6/2 = 3
Answer: the charge of the metallic Cr is 3+
d) MoO3: x + 3(2-) = 0 => x - 6 = 0 => x = 6
Answer: the charge of the metallic Mo is 6+
