Sub-surface mining involves digging tunnels and shafts to deep underground.
Answer: Option 4
Explanation:
Mining is the process of extracting ores of minerals from the earth crust depending upon the presence of ores in the earth crust position. Some mineral ores are tend to present in the surface of earth crust.
In order to extract them, surface mining is done. Strip mining and open pit mining are types of surface mining. This type of mining does not require tunnelling and digging deep shafts as the ores can be obtained from the upper layers of earth crust.
Smelting is the process of extracting minerals from the ores, so it is not a process of mining. Thus, sub-surface mining uses digging tunnels and shafts as they are required to extract ores buried in the deep underground of earth crust.