What should be used as a leavening agent for waffles if made several hours ahead of time?

Baking powder is the appropriate leavening agent for waffles that are prepared several hours in advance because it contains both an acid and a base that react when wet and then again when heated. This double-acting feature ensures that waffles rise during the cooking process, providing a light and fluffy texture.

When waffles are made ahead of time, using baking powder allows the batter to be mixed, rested, and then cooked without a significant loss in leavening power. This is particularly beneficial because the initial reaction occurs shortly after the wet ingredients are combined with the dry ingredients, while the second reaction occurs when the batter is heated.

Using yeast, while a possible option for leavening, requires more time to activate and would not be ideal if the batter is prepared too far in advance before cooking. Baking soda requires an acid to activate it, and it might not provide the necessary structure if the batter sits for a long time. Self-rising flour may seem convenient, but it is not as effective in providing the desired consistency and rise if the baking powder's effectiveness diminishes over time. Therefore, baking powder stands out as the most reliable choice for making waffles ahead of time.