The correct answer is: D. Tubewells
Gopalganj and Madhubani districts are located in the Ganges Plain, which is a region with a high water table. This makes tubewells an ideal means of irrigation, as they can access groundwater without the need for canals or other infrastructure.
Canals are a type of irrigation system that uses artificial waterways to transport water from a source, such as a river or lake, to fields. Canals are often used in areas with low rainfall, as they can provide a reliable source of water for crops. However, canals can be expensive to build and maintain, and they can also have a negative impact on the environment.
Ponds are another type of irrigation system that uses natural or artificial bodies of water to irrigate crops. Ponds are often used in areas with low rainfall, as they can provide a source of water for crops during dry periods. However, ponds can be expensive to build and maintain, and they can also have a negative impact on the environment.
Wells are a type of irrigation system that uses groundwater to irrigate crops. Wells are often used in areas with high water tables, as they can provide a reliable source of water for crops. However, wells can be expensive to drill and maintain, and they can also have a negative impact on the environment.
Tubewells are a type of irrigation system that uses deep wells to access groundwater. Tubewells are often used in areas with high water tables, as they can provide a reliable source of water for crops. Tubewells are also relatively inexpensive to drill and maintain, and they do not have a negative impact on the environment.
Therefore, the correct answer is: D. Tubewells