Which of the following does not show the law of conservation of mass?
a. 24g of mg burn in 32g o2 to produce 56g of mgo
b. 24ml of mg burn in 32ml o2 to produce 56ml of mgo
c. 2 atoms of mg react with 1 molecule of o2 to produce 2 units of mgo
d. 1 atom of mg reacts with 1 atom of o to produce a unit of mgo that contains 2 atoms

The answer is B because completely burning anything usually results in carbon being produced there for any gases such as oxygen which make up the molecule would be destroyed or recombined...since I know nothing about the environment in which it is burning such as open or closed vessel this suffices the question in assuming some mass would be lost as gases and/or not be part of the new molecule.