The law of conservation of mass states that mass is neither created nor destroyed in a chemical reaction. Therefore, the number of atoms of each element must be the same on both sides of the equation.
Let's analyze each option:
A. Mg + S → MgS2 - This equation does not balance because there are 2 sulfur atoms on the right side but only 1 on the left.
B. C + O2 → C2O - This equation does not balance because there are 2 carbon atoms on the right but only 1 on the left.
C. H2SO4 + Zn → 4ZnSO + H2 - This equation does not balance either, as the number of atoms on each side does not match.
D. 2H2 + O2 → 2H2O - This equation balances because there are 4 hydrogen atoms and 2 oxygen atoms on both sides.
Therefore, the correct option that demonstrates the law of conservation of mass is:
D. 2H2 + O2 → 2H2O