Which ingredient is primarily responsible for leavening in sweet dough?

Yeast is primarily responsible for leavening in sweet dough because it is a biological leavening agent that ferments and produces carbon dioxide gas when it consumes sugars. This carbon dioxide gas creates bubbles in the dough, causing it to rise and develop a light, airy texture. The fermentation process also contributes to flavor development and enhances the overall quality of the sweet dough, making it richer and more complex.

While butter provides richness and contributes to the overall texture and flavor of the dough, it does not leaven it. Sugar, although it can assist in yeast activation and contribute to flavor and browning, does not directly leaven the dough. Milk enriches the dough and can aid in browning and moisture retention, but it is not a leavening agent on its own. Hence, yeast stands out as the key ingredient that causes the dough to rise.