The correct answer is - A. replacing natural vegetation with cultivated crops.
The natural vegetation is perfectly adapted to a certain environment, and it is also able to keep the soil tight with its roots, so the erosion is minimal.
When the natural vegetation is replaced by cultivated crops, than a problem occurs in the erosion of the soil. That is because the cultivated crops, in general, do not have root systems that are as strong and widespread as the natural vegetation, thus the soil becomes loosened up, so it is much easier to be eroded.
