What must be true for a chemical reaction regarding atoms?

The correct answer reflects the law of conservation of mass, which states that matter cannot be created or destroyed in a chemical reaction. This means that all the atoms present in the reactants must be accounted for in the products. They can rearrange and form new substances, but the total number of atoms must remain the same before and after the reaction.

This principle ensures that there will always be a balance in chemical equations, where the number and type of atoms on the reactant side equal those on the product side. Understanding this law is fundamental to mastering chemical reactions, as it underlines why balancing equations is necessary in order to accurately represent what occurs during a reaction.