When yeast cells are added to wheat flour, bread becomes soft and porous because it produces CO2 and makes the bread spongy. During fermentation, yeast metabolizes sugars present in the dough, producing carbon dioxide gas as a byproduct. This gas forms bubbles throughout the dough, causing it to rise and creating a soft, airy texture in the finished bread. The expansion of the dough due to the production of carbon dioxide gas results in a porous structure, giving the bread its characteristic lightness and texture. This process, known as leavening, is essential for the formation of desirable bread characteristics. While yeast also produces other compounds like ethanol and organic acids during fermentation, it is the carbon dioxide gas generated by yeast metabolism that primarily contributes to the soft and porous texture of bread, making option B the correct answer.