Which of the following is NOT a common use of sugar in cooking?

The role of sugar in cooking encompasses various functions, but thickening sauces is generally not one of them. Sugar primarily serves as a sweetener in various dishes, enhancing flavors and balancing acidity. Additionally, in the context of preserving fruits, sugar acts as a preservative by drawing moisture out, which inhibits the growth of microorganisms.

In baking, sugar contributes to the overall structure and flavor of baked goods. It helps with the browning process, known as caramelization, which adds depth to the final product.

Conversely, sauces are typically thickened using ingredients like starches (such as flour or cornstarch), reduction methods, or emulsifying agents rather than sugar, which does not have the necessary properties to create a thicker consistency. Therefore, considering the distinct roles that sugar plays in cooking and its inability to effectively thicken sauces, it is clear why thickening sauces is not a common use of sugar.