Which of the following is a leavening agent used in baking?

Baking powder is a leavening agent used in baking because it contains a combination of an acid and a base that reacts when hydrated and heated, producing carbon dioxide gas. This gas gets trapped in the batter or dough, causing it to rise and become light and fluffy.

Understanding leavening agents is crucial in baking, as they contribute to the texture and volume of baked goods. Other items listed, such as vanilla extract, butter, and sugar, serve different purposes in baking. Vanilla extract is primarily used for flavoring, butter adds richness and moisture, and sugar not only sweetens but can also help with moisture retention and browning. However, none of these items have the leavening properties that baking powder provides.