Yeast is a eukaryotic and single-celled fungus. The commonly used species of yeast is Saccharomyces cerevisiae. It is also known as baker's yeast. The yeast is used in the production of fermented products like cakes, bread, and alcohol. The organism converts the fermentable sugars present in the substrate into carbon dioxide and ethanol. The carbon dioxide gas makes the dough of bread and cake soft and fluffy. There are pores in the bread and cakes which is due to the carbon dioxide gas bubbles which get trapped during the process of baking. Thus, the correct answer is option D.