What gas is produced when an acid reacts with a carbonate?

C. Carbon dioxide

When an acid reacts with a carbonate, the products are a salt, water, and carbon dioxide gas. A general example is:

$$\text{Acid} + \text{Carbonate} \rightarrow \text{Salt} + \text{Water} + \text{Carbon Dioxide}$$

For instance, hydrochloric acid reacting with calcium carbonate produces calcium chloride, water, and carbon dioxide:

$$\text{2HCl(aq)} + \text{CaCO}_3\text{(s)} \rightarrow \text{CaCl}_2\text{(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)}$$

The gas evolved is carbon dioxide.

The final answer is $\text{C. Carbon dioxide}$.