The correct answer is: a) Landslides.
Arunachal Pradesh falls within a seismically active zone, making it prone to landslides. Landslides are a major natural hazard in the state, and they can cause significant damage to infrastructure and loss of life. The state government has taken steps to mitigate the risk of landslides, but they remain a serious threat.
Tsunamis are not a major threat to Arunachal Pradesh, as the state is not located on the coast. However, the state is located near the Himalayas, which are a major earthquake zone. Earthquakes in the Himalayas can trigger landslides, which can then cause tsunamis.
Volcanic eruptions are also not a major threat to Arunachal Pradesh. The state is not located near any active volcanoes. However, there are some dormant volcanoes in the state, and there is a small risk that they could erupt in the future.
Heatwaves are not a major threat to Arunachal Pradesh. The state has a temperate climate, and temperatures rarely reach extreme levels. However, there is a small risk of heatwaves in the future, as the climate changes.
In conclusion, the correct answer is: a) Landslides.