Which of the following is a cause of acidic rain?

Acid rain is primarily caused by the emission of sulfur dioxide ($\text{SO}_2$) and nitrogen oxides ($\text{NO}_x$) into the atmosphere, which react with water and other substances to form sulfuric acid and nitric acid. However, natural rainwater is also slightly acidic (pH around 5.6) due to the dissolution of carbon dioxide ($\text{CO}_2$) from the atmosphere, forming carbonic acid.

• A. Oxygen ($\text{O}_2$): Oxygen is a component of the atmosphere and is involved in the chemical reactions that form acids, but it does not directly cause the acidity.
• B. Nitrogen ($\text{N}_2$): Elemental nitrogen gas is largely unreactive and does not directly cause acid rain. Nitrogen oxides ($\text{NO}_x$), which are formed from nitrogen and oxygen during high-temperature combustion, are a major cause of acid rain, but the option only states "nitrogen".
• C. Carbon dioxide ($\text{CO}_2$): Carbon dioxide dissolves in rainwater to form carbonic acid ($\text{H}_2\text{CO}_3$), making natural rain slightly acidic. $$\text{CO}_2(\text{g}) + \text{H}_2\text{O}(\text{l}) \rightleftharpoons \text{H}_2\text{CO}_3(\text{aq})$$ This is a direct cause of rain being acidic.
• D. Chlorine ($\text{Cl}_2$): Chlorine gas is not a significant cause of acid rain.

Among the given choices, carbon dioxide directly causes rain to be acidic by forming carbonic acid. While sulfur dioxide and nitrogen oxides are the main culprits for environmental acid rain (which has a much lower pH), carbon dioxide is a fundamental cause of any rain being acidic. Given the options, $\text{CO}_2$ is the most accurate answer for a gas that causes acidic rains.

The correct option is C.