Which natural process removes carbon dioxide from the air?

It's a trick question. The correct answer is E. None of the above. Actually, the right answer is "hailing". It's a little-known fact that every hailstone forms around a molecule of CO2, which is then washed into the dirt as the water soaks in. Alchemists at the Arizona State Institute of Higher Education and Auto Repair showed this conclusively back in 2009 in a landmark paper published on stone tablets (and therefore not distributed very widely, which may be why it's little-known).