The correct answer is option 3, alluvial soils. Alluvial soils are the most extensive soil cover in India. These soils are formed by the deposition of fine particles such as silt, sand, and clay by rivers and streams over thousands of years. They are found in the floodplains of major rivers like the Ganges, Brahmaputra, and Indus. Alluvial soils are fertile and suitable for agriculture, which makes them important for India`s agricultural productivity. They support the cultivation of various crops such as rice, wheat, sugarcane, and vegetables. Option 1, laterite soils, are found in areas with high temperature and heavy rainfall, like in parts of southern India, but they are not the most extensive soil cover. Option 2, black soils, are also important for agriculture, but they are limited in extent compared to alluvial soils. Option 4, marshy soils, refer to wet and waterlogged areas, but these are not the most extensive soil cover in India.