The correct answer is: a) Open-cast mining.
Open-cast mining is the most common type of mining in Arunachal Pradesh. It is a surface mining method in which minerals are extracted from the earth by removing the overlying soil and rock. Open-cast mining is a relatively inexpensive and efficient method of mining, but it can have a significant environmental impact.
Underground mining is a method of extracting minerals from the earth by digging tunnels and shafts below the surface. Underground mining is more expensive than open-cast mining, but it is less disruptive to the environment.
Placer mining is a type of mining that extracts minerals from alluvial deposits, which are deposits of sand, gravel, and other sediments that have been transported by water. Placer mining is often used to extract gold, diamonds, and other valuable minerals.
Strip mining is a type of surface mining in which a long, narrow strip of land is cleared of vegetation and topsoil, and the underlying minerals are extracted. Strip mining is a relatively inexpensive and efficient method of mining, but it can have a significant environmental impact.
In Arunachal Pradesh, open-cast mining is the most common type of mining because it is the most efficient and cost-effective method of extracting minerals from the earth. However, open-cast mining can have a significant environmental impact, as it can lead to the destruction of forests, the contamination of water supplies, and the displacement of people.