The region of rolling hills and valleys located in the south-central part of Assam is:

Karbi Anglong
Dima Hasao
Barak Valley
Bodoland

The correct answer is: a) Karbi Anglong.

Karbi Anglong is a district in the Indian state of Assam. It is located in the south-central part of the state, and is bordered by Nagaland to the north, Meghalaya to the east, and the districts of Golaghat and Jorhat to the west and south, respectively. The district has an area of 3,681 square kilometers (1,420 sq mi), and a population of 1,419,407 as of 2011. The district headquarters is Diphu.

Karbi Anglong is a hilly region, with an average elevation of 1,000 meters (3,300 ft). The main rivers in the district are the Dhansiri, the Kopili, and the Disang. The climate of Karbi Anglong is tropical, with a hot and humid summer and a cool and dry winter.

The main ethnic group in Karbi Anglong are the Karbis, who are a tribal people. The Karbis speak the Karbi language, which is a member of the Tibeto-Burman language family. The Karbis are predominantly Hindu, but there are also a significant number of Christians and Muslims in the district.

The economy of Karbi Anglong is based on agriculture, forestry, and mining. The main crops grown in the district are rice, maize, and tea. The forests of Karbi Anglong are home to a variety of wildlife, including elephants, tigers, and rhinoceroses. The district is also home to a number of coal mines.

Karbi Anglong is a beautiful and scenic region, with a rich history and culture. The district is a popular tourist destination, and is home to a number of historical and cultural sites.