Hint: Soil is a part of our ecosystem and is composed of organic matter, minerals, etc. Because of its composition, it can support and help in the growth of plants and also retain water. It takes years for soil formation and the climate of the region, topography, etc. all influence it.
Soil erosion refers to the removal or displacement of the top layer of the soil due to natural or man-made causes. Natural causes include water, wind, plants, and animals as well. Polluting the land and indiscriminate use of land resources by humans is one of the major causes of soil erosion.
The top-soil is rich in humus (organic matter), microorganisms, etc. The plant can derive its nutrients from the soil. One of the major effects or side effects of soil erosion is that the soil is rendered infertile. Soil erosion also leads to pollution and decreased water retention capacity, etc.
Trees and other plants have a strong root system. These roots hold the soil together and do not allow it to be displaced. Now animals, wild or part of the livestock, graze on these plants. When these animals (sometimes a large number of them) graze in the same region for a long period, they leave the soil bare and vulnerable to erosion. Also when these animals feed in the same region, chances are that their hoofs may pull out plants again leaving the soil bare.
Hence, the correct answer is option (B).
Note: Better livestock management can be one of the measures employed to prevent soil erosion due to overgrazing. The same grassland should not be frequently used for grazing; enough time should be given for the grass to regrow. In general soil erosion can be prevented by planting more trees in barren lands or places where trees have been cut down, better farming and agricultural practices, etc.