The correct answer is: d) Upper Assam
Deep tube wells are a major source of groundwater irrigation in Upper Assam. The region has a high water table and the wells are able to tap into this water source. The wells are used to irrigate crops such as rice, sugarcane, and vegetables.
Lower Assam is a low-lying region and the water table is not as high. As a result, deep tube wells are not as common in this region.
Barak Valley is a hilly region and the water table is even lower than in Lower Assam. As a result, deep tube wells are not used in this region at all.
Central Assam is a transitional region between Lower Assam and Upper Assam. The water table is higher in Central Assam than in Lower Assam, but not as high as in Upper Assam. As a result, deep tube wells are used in Central Assam, but not as commonly as in Upper Assam.