Based on the chemical equations provided, we can determine the reactants that would yield the given products. Here are the balanced chemical equations for each reaction:

MgO:
Mg + O2 = MgO

MgCl2:
Mg + Cl2 = MgCl2

Al2O3:
2Al + 3/2O2 = Al2O3

NaCl:
Na + Cl2 = 2NaCl

FeCl3:
2Fe + 3Cl2 = 2FeCl3

These equations show the reactants (on the left side of the equation) that would yield the corresponding products (on the right side of the equation).