Which of the following is NOT a leavening agent?

Flour is not a leavening agent; rather, it serves as the main structural component in baked goods. Leavening agents are substances used to produce gas in dough or batter, which causes it to rise and become light and porous.

Yeast, which is a living organism, ferments and produces carbon dioxide gas when it consumes sugars, resulting in a rise in bread and other yeast-leavened products. Baking powder contains both an acid and a base, releasing carbon dioxide when moistened and/or heated, which helps to leaven baked goods like cakes and cookies. Similarly, baking soda is a pure base that requires an acid to activate it, creating carbon dioxide gas that causes dough or batter to rise.

Flour, on the other hand, mainly contributes structure, flavor, and nutritional value to baked products but does not produce gas or cause rising by itself. Therefore, it is correctly identified as the option that does not function as a leavening agent.