The acid used in making baking soda and soft drink is

Baking powder is a mixture of sodium hydrogen carbonate (baking soda) and a solid acid. The acid is needed because sodium hydrogen carbonate only releases carbon dioxide gas, the substance that makes dough rise, when it reacts with an acid:

\[ \text{NaHCO}_3 + \text{acid} \rightarrow \text{CO}_2 + \text{H}_2\text{O} + \text{salt} \]

Tartaric acid, \( \text{C}_4\text{H}_6\text{O}_6 \), is a naturally occurring organic acid obtained mainly from grapes, and it is one of the classic solid acids used to formulate baking powder because it is stable when dry but dissolves and reacts quickly once water is added to the dough or batter. The same acid, because it has a pleasant sharp taste and is safe to consume in small amounts, is also added to soft drinks to give them their tart, refreshing flavour and to help balance the sweetness of the sugar in the drink.

The other acids listed do not fit both uses. Fatty acids are found in oils and fats and are not used as leavening or flavouring agents in drinks. Boric acid is toxic in the concentrations relevant to food and is not permitted as a food additive. Citric acid is common in fruit-flavoured drinks but is not the acid traditionally paired with sodium hydrogen carbonate in the classic baking-soda and soft-drink formulation being tested here.

When a question links a food acid to two different uses at once, check that the acid is both chemically suited to the reaction involved (reacting with a base to release gas) and safe and pleasant enough to be consumed directly, since not every food-grade acid satisfies both conditions.