The correct answer is: Alluvial soil.
Alluvial soil is a type of soil that is formed by the deposition of sediments from rivers and streams. It is typically rich in nutrients and organic matter, making it ideal for agriculture. Alluvial soils are found in many parts of the world, including the floodplains of major rivers, deltas, and coastal areas.
Red soil is a type of soil that is formed from the weathering of rocks that contain iron oxide. It is typically reddish-brown in color and is found in tropical and subtropical regions. Red soils are not as fertile as alluvial soils, but they can be used for agriculture if they are properly managed.
Black soil is a type of soil that is formed from the weathering of limestone and dolomite rocks. It is typically dark brown or black in color and is found in semi-arid and sub-humid regions. Black soils are very fertile and are well-suited for agriculture.
Mountain soil is a type of soil that is formed from the weathering of rocks that are found in mountains. It is typically thin and rocky, and it is not as fertile as other types of soil. Mountain soils are often used for grazing livestock or for growing crops that are tolerant of poor soil conditions.
In conclusion, alluvial soil is the most fertile type of soil and is the best suited for agriculture.