Which of the following must be present for baking soda to be an effective leavening agent?

Baking soda is a sodium bicarbonate, which requires an acid to activate its leavening properties. When baking soda is combined with an acidic ingredient, such as vinegar, lemon juice, or yogurt, a chemical reaction occurs that produces carbon dioxide gas. This gas creates air pockets in the batter or dough, allowing it to rise and become light and fluffy during the baking process.

In the absence of an acid, baking soda will not release carbon dioxide; instead, it may simply impart a bitter flavor without contributing to the leavening effect. Therefore, to ensure that baking soda functions effectively as a leavening agent, the presence of an acid is essential. This is why the correct response highlights the need for acid as a critical component in recipes that include baking soda.