The correct answer is - C. -1.9 C degrees.
The sea ice in the polar oceans needs little lower temperatures to start forming than what it would need if it was't forming on a saline water. The freshwater starts freezing as the temperature goes bellow 0 C degrees, but that is not the case with the saline water. The saline water, because of the salt that prevents the freezing to a certain point, needs temperature of -1.9 C degrees to start freezing. Once this temperature is reached, the salt can not prevent the freezing anymore, so the water on the polar oceans starts to freeze.