The correct answer is: a) Alluvial soil.
Alluvial soil is the most common type of soil found in Arunachal Pradesh. It is a type of soil that is formed by the deposition of sediments from rivers and streams. Alluvial soil is typically rich in nutrients and is well-drained, making it ideal for agriculture.
Red soil is a type of soil that is formed from the weathering of rocks that contain iron oxide. Red soil is typically found in tropical and subtropical regions. It is a good soil for growing crops, but it can be prone to erosion.
Mountain soil is a type of soil that is found in mountainous regions. It is a thin, nutrient-poor soil that is often rocky and acidic. Mountain soil is not well-suited for agriculture, but it can be used for grazing livestock.
Black soil is a type of soil that is found in semi-arid regions. It is a rich, dark soil that is high in organic matter. Black soil is ideal for growing crops, such as wheat and rice.