The correct answer is: a) Papum Pare.
Papum Pare is the most populous district in Arunachal Pradesh, with a population of 499,722 as of 2011. It is located in the central part of the state, and its headquarters is the town of Itanagar. The district is home to a number of different ethnic groups, including the Adi, Nyishi, Galo, and Apatani.
Upper Subansiri is the second most populous district in Arunachal Pradesh, with a population of 306,318 as of 2011. It is located in the eastern part of the state, and its headquarters is the town of Daporijo. The district is home to a number of different ethnic groups, including the Apatani, Nyishi, and Tagin.
Changlang is the third most populous district in Arunachal Pradesh, with a population of 255,387 as of 2011. It is located in the southeastern part of the state, and its headquarters is the town of Changlang. The district is home to a number of different ethnic groups, including the Singpho, Khampti, and Chakma.
East Kameng is the fourth most populous district in Arunachal Pradesh, with a population of 231,736 as of 2011. It is located in the northeastern part of the state, and its headquarters is the town of Seppa. The district is home to a number of different ethnic groups, including the Monpa, Sherdukpen, and Nyishi.