The correct answer is: All of the above.
The Imphal Valley is a fertile valley in the Indian state of Manipur. It is surrounded by hills and mountains, and has a number of rivers flowing through it. The valley is well-suited for agriculture, and a significant portion of the land is under irrigation.
The presence of rivers is one of the main reasons why the Imphal Valley is so well-suited for agriculture. The rivers provide water for irrigation, and also for drinking and other purposes. The government has also implemented a number of schemes to improve irrigation in the valley. These schemes have helped to increase the amount of land that is under irrigation, and have also made it easier for farmers to get access to water.
The fertile soils of the Imphal Valley are another reason why it is so well-suited for agriculture. The soils are rich in nutrients, and are ideal for growing a variety of crops. The climate of the valley is also ideal for agriculture, with warm temperatures and plenty of rainfall.
All of these factors have contributed to the fact that the Imphal Valley has a significant portion of land under irrigation. The presence of rivers, government schemes, and fertile soils have all played a role in making the valley a major agricultural region.