Answer : 4. "Methane"
Explanation :

Methane is the gas that can cause explosions in coal mines. Methane is a highly flammable gas, and when it accumulates in high concentrations in confined spaces like coal mines, it poses a significant risk. Methane explosions can occur if the gas comes into contact with an ignition source, such as a spark or flame, leading to a dangerous explosion. That's why proper ventilation and safety measures are crucial in coal mines to prevent methane buildup and potential explosions.