Which ingredient is commonly used to thicken Veloute?

Velouté is one of the five mother sauces in classical French cuisine, and it is typically thickened using a roux. A roux is a cooked mixture of equal parts flour and fat, usually butter, which is used to create a foundation for sauces. In the case of velouté, a white roux is commonly utilized. The flour in the roux serves as the thickening agent when it's blended with the stock, allowing for a smooth and velvety texture in the sauce.

While butter is an essential component of the roux, and flour plays a critical role in thickening sauces in general, the specific practice of making velouté relies primarily on the roux itself. Other options, such as egg yolks, are used to thicken other sauces like sabayon or sauces that require an emulsion, but they are not typical for velouté. Therefore, the use of a roux aligns perfectly with the traditional preparation method for velouté, making it the correct choice for this question.