Soil erosion can be prevented by

The correct option is Afforestation.

Soil erosion is the process by which the fertile topsoil is worn away or washed away by natural physical forces such as water and wind, or through human activities.
Let us understand why afforestation is the key method to prevent this process by examining the choices:

1. What is Afforestation?
Afforestation is the establishment of a forest or stand of trees in an area where there was no recent forest cover.
Trees and plants play a vital role in conserving soil in the following ways:
⇒ Root system binding: The extensive root networks of trees bind the soil particles tightly together, preventing them from being easily washed away by heavy rains or blown away by strong winds.
⇒ Canopy protection: The leaves and branches of trees reduce the direct impact of raindrops falling on the ground, thereby minimizing splash erosion.
⇒ Water absorption: Forests improve the soil's organic matter and structure, enhancing its capacity to absorb water and reducing surface runoff, which is a major driver of water erosion.

2. Why other options are incorrect:
⇒ Overgrazing: When livestock overgraze a land area, they destroy the grass and vegetation cover. Without plants to hold the soil, it becomes loose and highly vulnerable to erosion by wind and water.
⇒ Removal of vegetation: Similar to overgrazing, removing plants leaves the topsoil bare and exposed, accelerating soil erosion.
⇒ Increasing bird population: While birds are a crucial part of the ecosystem, they do not directly contribute to holding the physical structure of the soil together or protecting it from wind and water currents in a significant way.

Therefore, planting trees (Afforestation) is the most effective and direct method to prevent soil erosion.