The mining which is uneconomic if beds are deeper depth.

Solution (By Examveda Team)

Opencast mining is a surface mining technique where the overburden (the layer of rock and soil above the mineral deposit) is removed to expose the ore body. It's cost-effective for shallow deposits.

Option A: Open cast
Opencast mining becomes uneconomical at greater depths because the cost of removing the increasing volume of overburden outweighs the value of the extracted ore. The deeper the ore body, the more overburden needs to be excavated, transported, and disposed of, significantly increasing the cost. This makes it an uneconomical choice for deep deposits.

Option B: Long wall
Longwall mining is an underground mining method. It's used for extracting relatively flat, tabular deposits of coal or other minerals. Depth isn't the primary factor limiting its economic viability; rather factors such as geological conditions, roof stability, and the thickness and quality of the seam play a more significant role. Therefore, it is not inherently uneconomical at deeper depths compared to opencast.

Option C: Bord and Pillar
Bord and pillar is another underground mining technique where pillars of ore are left in place to support the roof. Again, the economic viability of this method is mostly determined by factors other than depth, though the costs associated with deeper shafts and haulage could eventually make it less profitable at extreme depths.

Correct Answer: A
The correct answer is Option A: Open cast because the costs associated with removing the progressively larger volume of overburden at increased depths render opencast mining uneconomical beyond a certain depth. Options B and C are underground methods and are not primarily limited by depth in the same way.