What can cause damage to an engine's cooling system in winter?

You answered correctly. Insufficient frost protection can damage the engine's cooling system in winter because water in the cooling system can freeze at low temperatures. When coolant freezes, it expands, which can crack hoses, the radiator, or even the engine block. That's why you always need the right mix of antifreeze in the coolant, especially in cold weather. The other answers are incorrect: it is not necessary to change the coolant twice a year, and even closed systems still require maintenance like checking antifreeze levels. Remember to regularly check frost protection before winter to prevent costly engine damage.