The mass of the reactants of a chemical reaction will be the mass of the products.
a less than
b greater than
c equal to
d disproportionate to

 [C]:  "equal to" .

     "The mass of the reactants of a chemical reaction will be  equal to   the mass of the products."

             There is more space between gas particles than the size of the particles.

Explanation:
                   This scenario can be understand by taking a very simple example. As we know that 1 mole of any gas at standard temperature and pressure occupy 22.4 liters of volume. Lets take Hydrogen gas and Oxygen gas, 1 mole of each gas will occupy same volume. Why it is so? Why same volume although Oxygen is 16 times more heavier? This is because the space between gas molecules is very large. Approximately the distance between gas molecules is 300 times greater than their own diameter from its neighbor molecules.