The correct answer is - d. Cold air chills water molecules.
When the ocean currents from the lower latitudes reach the polar regions, the water starts to change its properties. First, because of the cold air, the water starts to chill. After the cold air starts to chill the water, the water molecules start to move closer to one another. Than, as the water becomes colder and denser, it starts to sink deeper, and starts to move towards the lower latitudes in the form of a deep, cold ocean current.