The correct answer is: A. Purnea and Saharsa
Khadar soil is a type of alluvial soil that is found in the floodplains of rivers. It is a fertile soil that is well-drained and has a high water retention capacity. Khadar soil is ideal for growing rice, sugarcane, and other crops.
The districts of Purnea and Saharsa are located in the state of Bihar, India. These districts are located in the Ganges plain, which is a region that is characterized by its alluvial soil. The alluvial soil in these districts is a type of khadar soil.
The districts of Patna and Nalanda are also located in the state of Bihar. However, these districts are located in the central part of the state, which is a region that is characterized by its red soil. The red soil in these districts is not a type of khadar soil.
The districts of Bhagalpur and Araria are also located in the state of Bihar. However, these districts are located in the eastern part of the state, which is a region that is characterized by its laterite soil. The laterite soil in these districts is not a type of khadar soil.
Therefore, the districts with khadar soil spread are Purnea and Saharsa.