The correct answer is: b) Southern plains.
The southern plains of Arunachal Pradesh are the most vulnerable to floods. This is because the southern plains are located in the Brahmaputra River basin, which is a major floodplain. The Brahmaputra River is one of the most powerful rivers in the world, and it is prone to flooding during the monsoon season. The southern plains are also located in a seismically active region, which means that they are at risk of earthquakes that can trigger landslides and floods.
The northern mountainous areas of Arunachal Pradesh are also vulnerable to floods, but they are less vulnerable than the southern plains. This is because the northern mountainous areas are located in a less populated region, and they are less developed. The northern mountainous areas are also less likely to be affected by earthquakes.
The high altitude regions of Arunachal Pradesh are the least vulnerable to floods. This is because the high altitude regions are located in a mountainous region, and they are not located in a floodplain. The high altitude regions are also less likely to be affected by earthquakes.
The western border regions of Arunachal Pradesh are also vulnerable to floods, but they are less vulnerable than the southern plains. This is because the western border regions are located in a less populated region, and they are less developed. The western border regions are also less likely to be affected by earthquakes.