Final answer:
Option D, 2Mg + O2 --> 2MgO, is the correct balanced equation for the burning of magnesium in the presence of oxygen, reflecting the stoichiometry of the reaction.
Step-by-step explanation:
The balanced equation for the burning of magnesium in the presence of oxygen is D. 2Mg + O2 --> 2MgO. This reaction is a combination reaction where magnesium is oxidized as it reacts with oxygen to form magnesium oxide.
Because oxygen is a diatomic molecule, two magnesium atoms will be needed to combine with one oxygen molecule, which consists of two oxygen atoms, ensuring mass balance and adherence to the Law of Conservation of Mass.