The correct answer is: A) Northern plains.
Black soils are predominantly found in the northern plains of Karnataka. These soils are rich in iron and aluminum, and are known for their fertility. They are well-suited for growing crops such as rice, sugarcane, and cotton.
The southern districts of Karnataka are mostly covered by red soils. These soils are poor in nutrients, and are not as well-suited for agriculture as black soils.
The coastal belt of Karnataka is characterized by sandy soils. These soils are not very fertile, and are mostly used for growing crops such as coconut, cashew, and arecanut.
The hilly regions of Karnataka are covered by a variety of soils, including red soils, black soils, and laterite soils. These soils are not as well-suited for agriculture as the soils found in the northern plains, but they are used for growing a variety of crops, including coffee, tea, and spices.