Option (B) is correct because primitive subsistence agriculture is also slash and burn agriculture. It is widely practised by many tribes in the tropics especially in Africa, south East Asia.

Option (A) is not correct because extensive subsistence agriculture is practised in moderately populated region and it is practised in large farms with relatively lower inputs.

Option (C) is not correct because extensive commercial grain cultivation is the cultivation of principal crop such as wheat and also other crops like corn, barley etc. It is practised in semi arid region.

Option (D) is not correct because mixed farming is also known as polyculture. It is a system of farming which involves the growing of crops as well s the raising of livestock.