Right answer: warm air from the south rises
An updraft (also called updraufgt in UK) is an upward air or gas current.
This is due differences in temperature between two masses of air.
How does it work?
A good example is the surface air in the ground that is heated, making this air warmer than the air above. Now, warmer air is less dense, so it rises and is replaced by descending cooler air creating a heat transfer and differences in temperature.
Therefore the right answer is C.
