The systematic name of Mg(NO3)2 is magnesium nitrate. If you were confused, there are two of the nitrate because magnesium has a charge of 2+ while just one nitrate has a 1- charge. They would have to cancel out because it is an ionic compound.
Magnesium Nitrate is the correct answer because mg only has one atom while nitrogen contains two atoms because 2x1=2 and oxygen contains 6 because 2 x 3= 6
