Concept:Alcoholic fermentation of sugar produces ethanol and carbon(IV) oxide as the main products.Explanation:During fermentation, yeast breaks down glucose into ethanol and carbon(IV) oxide gas.The simplified equation is:C6H12O6yeast2C2H5OH+2CO2Here, ethanol is the main liquid product, while carbon(IV) oxide, CO2, is released as the by-product.Ethanoic acid is formed by further oxidation of ethanol, not as a direct by-product of fermentation.Propanol and propan-1,2,3-triol are not produced from sugar fermentation.Therefore, the by-product asked in the question is carbon(IV) oxide.Answer:A. carbon(IV) oxide.