The correct answer is: a) Upper Assam
Upper Assam is the region of Assam that receives the highest rainfall. The average annual rainfall in Upper Assam is about 3,000 mm. The main reason for the high rainfall in Upper Assam is the presence of the Eastern Ghats mountain range. The Eastern Ghats block the moisture-laden winds from the Bay of Bengal, which causes the winds to rise and release their moisture as rain.
The other options are incorrect because they do not receive as much rainfall as Upper Assam. Lower Assam receives an average annual rainfall of about 2,000 mm, Barak Valley receives an average annual rainfall of about 1,500 mm, and Karbi Anglong Hills receives an average annual rainfall of about 1,000 mm.
Upper Assam is a fertile region and is home to a number of important rivers, including the Brahmaputra, the Dibang, and the Lohit. The region is also home to a number of important wildlife sanctuaries, including the Kaziranga National Park and the Manas National Park.