Which of the following is a common method for producing sweet wines?

Using grapes concentrated in sugar is a common method for producing sweet wines because it enhances the natural sweetness of the wine. This concentration can be achieved through various techniques, such as selecting very ripe grapes, allowing water to evaporate from the grapes to concentrate their sugars (a process known as 'passerillage'), or employing methods like noble rot, where the Botrytis cinerea fungus selectively dehydrates grapes, intensifying their sugar content and flavor profile. By starting with grapes that already have a high sugar content, winemakers can create wines that finish with elevated sweetness after fermentation.

While increasing fermentation by adding yeast is an essential step in winemaking, it does not directly create sweetness; instead, it transforms sugars into alcohol. Adding carbon dioxide during fermentation relates more to creating sparkling wines rather than enhancing sweetness. Finally, harvesting grapes at a lower sugar level would likely produce dry wines or wines with less sweetness, which is opposite the aim of creating sweet wines. Hence, the approach of using grapes concentrated in sugar is the most appropriate method to achieve the desired level of sweetness in the final product.