The correct answer is A. Carbon dioxide.
Carbon dioxide is a gas that is naturally present in the atmosphere. It is also released when we breathe, when plants decay, and when fossil fuels are burned. When carbon dioxide dissolves in water, it forms carbonic acid. Carbonic acid is a weak acid, but it can still cause water to become acidic.
Oxygen is a gas that is also naturally present in the atmosphere. It is essential for life, and it is also used in many industrial processes. Oxygen does not cause water to become acidic.
Hydrogen is a gas that is found in water. It is also produced when water is electrolyzed. Hydrogen does not cause water to become acidic.
Nitrogen is a gas that is found in the atmosphere. It is also found in many fertilizers and explosives. Nitrogen does not cause water to become acidic.
In conclusion, the most common cause of acidity in water is carbon dioxide.