The correct statement that explains why the student's claim is incorrect is:
• The reaction produced a gas that escaped the beaker.
This is because if a gas was released during the mixing of the liquids, some of the mass would have escaped into the environment, leading to a lower measured mass of the combined liquids in the beaker. This does not indicate that matter is not conserved; rather, it shows that mass was lost from the system due to the release of gas.