The correct answer is D. pedogenesis.
Pedogenesis is the process of soil formation. It is a complex process that is influenced by many factors, including climate, parent material, topography, organisms, and time.
Leaching is the process by which water moves through soil, carrying dissolved minerals and other materials with it.
Illuviation is the process by which clay and other fine particles are transported from the upper layers of soil to the lower layers.
Eluviation is the process by which soluble materials are transported from the upper layers of soil to the lower layers.
Pedogenesis is a slow process that can take thousands of years to complete. It is an important process because it creates the soil that plants need to grow.