The correct answer is: A. North-West Alluvial Plain
The North-West Alluvial Plain is the most fertile land of Bihar. It is a vast plain that is formed by the confluence of the Ganges, Yamuna, and Son rivers. The soil in this region is rich in nutrients and is ideal for growing a variety of crops. The North-West Alluvial Plain is also home to a number of important cities, including Patna, the capital of Bihar.
The North-East Alluvial Plain is also a fertile region, but it is not as fertile as the North-West Alluvial Plain. The soil in this region is not as rich in nutrients and is not as well-drained. The North-East Alluvial Plain is also home to a number of important cities, including Bhagalpur and Darbhanga.
The South-Bihar Alluvial Plain is the least fertile region of Bihar. The soil in this region is not as rich in nutrients and is not as well-drained. The South-Bihar Alluvial Plain is also home to a number of important cities, including Gaya and Ranchi.