The correct answer is C) accidental breakage can cause severe sparking and explosions. The reasoning is that a mercury-filled thermometer is an electrical conductor, and if it were to break inside a lead-acid battery, the mercury could come into contact with the battery terminals. This would create a conductive path between the positive and negative terminals, potentially causing a short circuit and sparking. The sparks, in the presence of flammable hydrogen gas typically present around lead-acid batteries, could then ignite the gas and lead to an explosion. The other options are incorrect because they do not address the primary hazard of the mercury-filled thermometer - the risk of sparking and explosion. While accidental breakage could potentially cause other issues like corrosion or gassing, the main concern is the fire and explosion risk from the electrical short circuit.