The correct answer is: c) Surface irrigation.
Surface irrigation is the most commonly used irrigation method in Assam. It is a method of irrigation in which water is applied to the surface of the soil, either by gravity or by pumping. Surface irrigation is the simplest and most economical method of irrigation, but it is also the least efficient.
Drip irrigation is a method of irrigation in which water is applied directly to the roots of plants, usually through a network of small pipes or tubes. Drip irrigation is a more efficient method of irrigation than surface irrigation, but it is also more expensive.
Sprinkler irrigation is a method of irrigation in which water is sprayed onto the surface of the soil from above, usually through a network of sprinklers. Sprinkler irrigation is a more efficient method of irrigation than surface irrigation, but it is also more expensive.
Canal irrigation is a method of irrigation in which water is transported from a source, such as a river or a reservoir, to fields through a network of canals. Canal irrigation is a very efficient method of irrigation, but it is also very expensive.
In Assam, surface irrigation is the most commonly used irrigation method because it is the simplest and most economical method. However, surface irrigation is also the least efficient method of irrigation, and it can be wasteful of water. In recent years, there has been a trend towards using more efficient methods of irrigation, such as drip irrigation and sprinkler irrigation. However, these methods are more expensive than surface irrigation, and they are not yet widely used in Assam.