The correct answer is (a) Fragile Himalayan terrain.
Arunachal Pradesh is located in the eastern Himalayas, which is a region of high seismic activity. The Himalayas are a young mountain range that is still being formed by the collision of the Indian and Eurasian tectonic plates. This collision causes the land to rise, which puts a lot of stress on the rocks. When the rocks reach their breaking point, they can collapse, causing landslides.
Coastal erosion is the process of land being worn away by the action of waves and tides. This is a natural process that occurs all over the world, but it is more severe in some places than others. Coastal erosion can be caused by a number of factors, including storms, rising sea levels, and human activities such as development and agriculture.
The presence of deserts does not make a place prone to landslides. In fact, deserts are often very stable because there is little vegetation to hold the soil in place.
Volcanic activity can cause landslides, but it is not the main cause of landslides in Arunachal Pradesh. The Himalayas are not a volcanic region, and there are no active volcanoes in Arunachal Pradesh.
In conclusion, the correct answer is (a) Fragile Himalayan terrain.