What type of microorganism is mainly responsible for bread-making?

The correct choice is yeast, which plays a crucial role in the bread-making process. Yeasts are single-celled fungi that utilize sugars in the dough during fermentation. This process produces carbon dioxide and alcohol, which causes the dough to rise and develop the light, airy texture characteristic of bread.

In this context, yeasts are specifically employed for their unique ability to ferment carbohydrates, leading to the leavening of dough. The carbon dioxide gas produced during fermentation gets trapped in the dough's structure, creating the desired volume and texture in baked bread.

While molds, another type of fungi, can also be involved in food processing and spoilage, they do not play a direct role in the leavening process of bread. Bacteria are used in other fermentation processes, such as in yogurt and some types of sourdough, but they do not produce the same rise in bread leavening as yeasts do. Fungi encompasses both yeasts and molds, but in the context of bread-making, yeast is the specific microorganism that is primarily responsible for the leavening action.