Correct answer: a base
Explanation:
An antacid is a substance that is used to neutralize excess acid in the stomach. Since it neutralizes acid, it must be a base.
Common antacids include:
- Magnesium hydroxide \(\text{Mg(OH)}_2\)
- Aluminium hydroxide \(\text{Al(OH)}_3 \)
- Sodium bicarbonate \( \text{NaHCO}_3 \)
Hence, the correct answer is: a base.