Which best explains why the total mass of the product(s) would be less than the
total weight of the reactant(s) after a chemical reaction?

The law of conservation of mass applies to all chemical reaction, so this means that there was equal mass before and after the reaction. However, if the mass has decreased, then it means there has been a loss of substance. The most common cause of this is the liberation of gases from a chemical reaction mixture, the mass of which is not measured by the scale.