So a hydrocarbon was combusted, and the product mass, that of carbon dioxide and water, was equal to the reactant mass. Mass was conserved, as it is in every chemical reaction.
We can represent this by means of stoichiometrically balanced equation, which also conserves mass and charge:
And clearly the products and reactants were in stoichiometric proportion. Of course, I have done a lot of work for a multiple choice question. The simple fact is that mass is conserved for EVERY chemical reaction. You start with