The correct answer is: b) Heavy rainfall.
Heavy rainfall is the main source of erosion in Assam. The state receives an average annual rainfall of about 3,000 mm, which is much higher than the national average of about 1,200 mm. This high rainfall causes the soil to become saturated and lose its cohesion, making it more susceptible to erosion.
The other options are incorrect because:
- Wind erosion is not a major problem in Assam. The state is located in a tropical climate, which means that the wind speeds are not high enough to cause significant erosion.
- Earthquakes are also not a major problem in Assam. The state is located in a seismically active region, but the earthquakes that do occur are usually of low magnitude and do not cause significant damage.
- Deforestation is a problem in Assam, but it is not the main source of erosion. Deforestation can lead to soil erosion, but it is not the only factor that contributes to erosion. Other factors, such as heavy rainfall, can also contribute to erosion.
In conclusion, the main source of erosion in Assam is heavy rainfall. The state receives an average annual rainfall of about 3,000 mm, which is much higher than the national average of about 1,200 mm. This high rainfall causes the soil to become saturated and lose its cohesion, making it more susceptible to erosion.