The principle of conservation of mass states that the total mass of the reactants in a chemical reaction must equal the total mass of the products. In Stephanie's experiment, she starts with 10.0 grams of zinc and 25.0 grams of bromine, which together total 35.0 grams.
Therefore, the option that most strongly substantiates her assertion of mass conservation is:
B. The mass of the final product will be equal to the total mass of both reactants, 35.0 grams.
This option accurately reflects the principle of conservation of mass in a chemical reaction.