What is one thickening agent commonly used in French cuisine?

Roux is a fundamental thickening agent in French cuisine, made by cooking equal parts of flour and fat, typically butter, together until they form a smooth paste. This technique not only helps to thicken sauces but also adds a complex flavor and richness to dishes. The base can be cooked to varying degrees, resulting in a white roux for lighter sauces, a blond roux for sauces like velouté, or a dark brown roux for richer sauces like gumbo. Its versatility and ability to create a stable emulsion make it a staple in both classic and modern French cooking.

While gelatin is used for gelling and stabilizing certain dishes, it does not serve as a thickening agent in the same way roux does. Baking powder is a leavening agent used primarily in baked goods rather than as a thickener, and starch, while a valid thickener in various cuisines, is not as traditionally entrenched in French culinary techniques as roux. Therefore, the importance of roux in French cooking makes it the correct choice for this question.