The correct answer is D. Methane.
Methane is a colorless, odorless, and flammable gas that is found in coal mines. It is produced by the decomposition of organic matter, such as plant and animal remains. Methane can accumulate in coal mines and can be ignited by a spark or flame, causing an explosion.
Hydrogen is a colorless, odorless, and flammable gas that is also found in coal mines. However, it is less common than methane and is not as likely to cause explosions.
Carbon monoxide is a colorless, odorless, and poisonous gas that is also found in coal mines. It is produced when coal is burned. Carbon monoxide can be fatal if inhaled in high concentrations.
Air is a mixture of gases, including nitrogen, oxygen, argon, and carbon dioxide. It is not flammable and does not cause explosions.
In conclusion, the gas that usually causes explosions in coal mines is methane.