Which of the following compounds are administered as antacids?
(a) Sodium carbonate
(b) Sodium hydrogencarbonate
(c) Aluminium carbonate
(d) Magnesium hydroxide

Answers: (b) Sodium hydrogen carbonate, and (d) Magnesium hydroxide

The antacid is a substance which neutralizes excess of acid in the stomach.
The chemical formula of sodium hydrogen carbonate is $NaHC{O}_3$. It is often used as an antacid because it neutralizes excess of HCl secreted from the walls of the stomach
$NaHC{O}_3\left(s\right)+HCl\left(aq\right)\to NaCl\left(aq\right)+H_{2}0\left(l\right)+C{O}_2$
However, excessive hydrogen carbonate can make the stomach alkaline and trigger the production of even more acid.
Milk of magnesia, which is an aqueous solution of magnesium hydroxide is also used as an antacid.