In order to predict the formula of an ionic compound, we need to know the charges that they have, because an ionic compound must be neutral in the total charge of the compound.
B. Sulfate, SO4^-2, has a charge of -2, Rubidium has a charge of +1, therefore the chemical formula that will correctly neutralize the charges will be:
Rb2SO4
C. The same principle for Magnesium acetate, Magnesium has a +2 charge and acetate has a -1 charge, therefore we will need 2 acetates:
Mg(CH3COO)2
D. Strontium bromide will have the chemical formula: SrBr2
E. Calcium fluoride will have the chemical formula: CaF2