The correct answer is: A. Jamui
Jamui is a district in the Indian state of Bihar. It is one of the 38 districts of Bihar and is located in the south-central part of the state. The district headquarters is Jamui town.
Jamui is a major producer of masoor, a type of lentil. The district has a long history of masoor cultivation and is known for its high-quality masoor. Masoor is a major source of income for the people of Jamui and is an important part of the local economy.
The other options are incorrect because they are not major producers of masoor. Sheikhpura, Patna, and Madhepura are all districts in Bihar, but they do not produce as much masoor as Jamui.
In conclusion, the chief producer of masoor is Jamui.