The correct answer is: B. Less than the saturation vapour pressure over ice.
Saturation vapour pressure is the maximum amount of water vapour that can be present in the air at a given temperature and pressure. When the air temperature is below freezing, the water molecules have less energy and are less likely to escape from the liquid state. This means that the saturation vapour pressure over water is less than the saturation vapour pressure over ice.
Option A is incorrect because the saturation vapour pressure over water is never equal to zero. Even at very low temperatures, there is always some water vapour present in the air.
Option C is incorrect because the saturation vapour pressure over water is always less than the saturation vapour pressure over ice. This is because water molecules have less energy at lower temperatures and are less likely to escape from the liquid state.
Option D is incorrect because the saturation vapour pressure over water is never equal to the saturation vapour pressure over ice.