Which of the following is a critical factor in determining the sweetness of a wine?

The critical factor in determining the sweetness of a wine is its residual sugar content. Residual sugar refers to the sugars that remain in the wine after fermentation, which contributes directly to the wine's perceived sweetness. When yeast converts grape sugars into alcohol, some of those sugars may not be fully fermented, and the amount left over is what gives the wine its sweetness.

While other factors like alcohol level and acidity can influence how sweetness is perceived, they do not directly contribute to the actual sugar content of the wine. For example, a wine with higher acidity can taste less sweet even if it has the same amount of residual sugar compared to a sweeter wine with lower acidity. Likewise, tannins can provide astringency and structure in red wines but do not affect sweetness directly, which makes them less relevant when assessing sweetness in a wine. Thus, understanding residual sugar is essential to accurately gauge a wine's sweetness level.