Given what we know, we can confirm that the best way to prove that no mass was lost during the reaction would be to follow option C and determine the masses of all substances before and after the reaction.
This method provides two sets of data in order to draw a comparison. The other option will either not produce comparable data, or do not take into account all of the elements being produced.
Therefore, by comparing the mass of baking soda and vinegar before the reaction to the mass of carbon dioxide and water after the reaction, we can analyze the differences if any, and confirm that no mass was lost during the reaction.
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