The correct answer is: b) Plain areas and foothills.
Arunachal Pradesh is a state in northeastern India. It is the largest state in the country by area, but has a relatively small population. The state is located in the Himalayas, and its terrain is mostly mountainous. The population of Arunachal Pradesh is concentrated in the plain areas and foothills of the state. These areas are more accessible and have more fertile land than the mountainous regions. They are also home to the state’s major cities, such as Itanagar, the capital.
The high-altitude mountainous regions of Arunachal Pradesh are sparsely populated. These regions are difficult to access and have a harsh climate. The remote border regions of Arunachal Pradesh are also sparsely populated. These regions are located near the borders of China and Myanmar, and are often subject to conflict. Areas with mining activities are also sparsely populated. These areas are often located in remote areas, and the mining activities can have a negative impact on the environment.
In conclusion, the districts in Arunachal Pradesh with high population density tend to be located in the plain areas and foothills of the state. These areas are more accessible and have more fertile land than the mountainous regions. They are also home to the state’s major cities.