The balance chemical reaction describing the reaction between Mg2+ and Cl- is shown below,
Mg2+ + 2Cl- ----> MgCl2
This is a reaction where there is a transfer of electrons. Since the valence electron of Mg2+ is 2 and that of Cl- is only 1. There is a need for 2 Cl- to react with 1 Mg2+ in order to balance the equation.
Thus, the anwer is MgCl2.