The correct answer is A. Saran, Siwan, Gopalganj.
Ahar-pyne is a traditional irrigation method used in the Gangetic plains of India. It consists of a network of small reservoirs (ahars) and channels (pynes) that are used to store and distribute water during the dry season. The ahars are typically built on the banks of rivers or streams, and the pynes are dug to connect them to the fields that need to be irrigated.
Ahar-pyne irrigation is a very efficient way to use water, and it is particularly well-suited to the dry climate of the Gangetic plains. It is also a very sustainable method of irrigation, as it does not require the use of any chemicals or fertilizers.
The districts of Saran, Siwan, and Gopalganj are located in the eastern part of the Gangetic plains, and they are all major rice-producing areas. Ahar-pyne irrigation is widely used in these districts, and it is estimated that it irrigates over 2 million hectares of land.
The other options are incorrect because they do not include any districts that are located in the eastern part of the Gangetic plains.