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 sand, gravel, or other loose material. 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 removed to expose the mineral deposit below. 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 and other natural habitats.