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 overlying soil and rock. Open-cast mining is a relatively inexpensive and efficient way to extract minerals, but it can have a significant environmental impact.
Underground mining is a type of mining in which minerals are extracted from beneath the surface of the earth. Underground mining is more expensive and time-consuming than open-cast mining, but it can be used to extract minerals that are not accessible by other methods. Underground mining can also have a significant environmental impact, as it can cause subsidence and water pollution.
Placer mining is a type of mining in which minerals are extracted from alluvial deposits, which are deposits of sand, gravel, and other sediments that have been transported by water. Placer mining is a relatively low-impact type of mining, but it can be expensive and time-consuming.
Strip mining is a type of surface mining in which a long, narrow strip of land is excavated to extract minerals. Strip mining is a relatively inexpensive and efficient way to extract minerals, but it can have a significant environmental impact.