The item that is a salt among the options you provided is calcium carbonate.
Here’s a brief explanation of each option:
- Antacid: This is a term for a group of substances that can neutralize stomach acidity, and it often contains various compounds, some of which can be salts, but it is not specifically a salt itself.
- Calcium carbonate: This is a salt that is formed from the reaction of calcium (a base) with carbonic acid (an acid). It is often used in antacids and as a dietary calcium supplement.
- Sodium hydroxide: This is a strong base (not a salt) and is typically used in various industrial applications and cleaning agents.
- Carbonic acid: This is an acid (H2CO3) and is not a salt.
Thus, the correct answer is calcium carbonate.