The correct answer is c) River deposition.
Alluvial soil is a type of soil that is formed by the deposition of sediment from rivers. It is typically found in floodplains and deltas. Alluvial soils are very fertile and are often used for agriculture.
Volcanic eruptions are a process that occurs when molten rock, or magma, rises to the surface of the Earth and erupts through a volcano. Volcanic eruptions can cause the formation of new land, but they do not typically contribute to the formation of new alluvial soils.
Earthquakes are a process that occurs when the Earth’s crust moves. Earthquakes can cause the formation of new land, but they do not typically contribute to the formation of new alluvial soils.
Coastal erosion is a process that occurs when waves and currents erode the coastline. Coastal erosion can cause the loss of land, but it does not typically contribute to the formation of new alluvial soils.
In conclusion, the process that contributes significantly to the formation of new alluvial soils in Assam is river deposition.