The correct answer is: a) Earthquakes.
Arunachal Pradesh is located in the eastern Himalayas, which is a seismically active region. The state has experienced several earthquakes in recent years, including a magnitude 6.8 earthquake in 2011 that killed at least 11 people.
Tsunamis are not a major threat to Arunachal Pradesh, as the state is located far from the Indian Ocean. However, the state is located in the path of the Brahmaputra River, which could be affected by a tsunami from a distant earthquake.
Cyclones are also not a major threat to Arunachal Pradesh, as the state is located in the rain shadow of the Himalayas. However, the state is located in the path of the monsoon, which can bring heavy rains and flooding.
Volcanic eruptions are not a major threat to Arunachal Pradesh, as the state is not located in a volcanic region. However, there are some dormant volcanoes in the state, and there is a small risk of an eruption in the future.
In conclusion, Arunachal Pradesh is prone to earthquakes due to its geological location. The state has experienced several earthquakes in recent years, and there is a risk of future earthquakes.