When yeast cells are O2 starved, fermentation serves as the source of energy. This results in the production of :

When yeast cells experience oxygen-starved conditions, they undergo anaerobic respiration, specifically a process known as alcoholic fermentation. During this process, glucose is first broken down into pyruvate through glycolysis, which generates a small amount of energy in the form of ATP.

In the absence of oxygen, yeast further converts pyruvate into Ethanol and Carbon Dioxide (CO2). This biochemical pathway is essential for regenerating NAD+, allowing glycolysis to continue and providing the cell with a steady, albeit lower, supply of energy. While lactic acid fermentation occurs in animal muscle cells under similar anaerobic conditions, yeast is characterized by the production of ethanol and CO2. Thus, the final products of fermentation in yeast are ATP, CO2, and Ethanol, aligning perfectly with Option A.