Which of the following can be used as an ingredient in antacids? 

Baking soda

The correct answer is option 3. Baking soda.

Baking soda, also known as sodium bicarbonate \((NaHCO_3)\), is commonly used as an ingredient in antacids. Antacids are medications that help neutralize stomach acid to relieve symptoms of heartburn, indigestion, and acid reflux.

Baking soda works as an antacid because it is a basic compound. When it reacts with the hydrochloric acid \((HCl)\) present in the stomach, it forms water, carbon dioxide, and sodium chloride (table salt). This reaction helps to neutralize excess stomach acid, reducing the discomfort associated with acid reflux and indigestion.

The chemical reaction between baking soda and stomach acid can be represented as follows:

\(\underset{\underset{bicarbonate}{Sodium}}{NaHCO_3} + \underset{\underset{acid}{Hydrochloric}}{HCl} \longrightarrow \underset{\underset{chloride}{Sodium}}{NaCl} + \underset{Water}{H_2O} + \underset{\underset{dioxide}{Carbon}}{CO_2}\)

As a result, baking soda can effectively alleviate symptoms of acid-related gastrointestinal issues when used as an ingredient in antacid formulations.