What type of sauce is typically made by thickening stock with a roux?

The correct choice is the sauce typically made by thickening stock with a roux, which is indeed velouté. Velouté sauce is one of the five mother sauces in classical French cuisine. It is created by combining a pale stock, such as chicken, fish, or veal stock, with a roux, which is a mixture of equal parts flour and fat cooked together. This combination results in a smooth, silky sauce that serves as a base for many other sauces.

Béchamel is another mother sauce, but it is made by thickening milk with a roux, not stock. Hollandaise sauce is an emulsified sauce made with egg yolks, butter, and lemon juice; it does not involve thickening stock. Gravy generally refers to a sauce made from the juices of cooked meat, often thickened with a roux, but it originates from the drippings rather than stock. Thus, the unique characteristics and method of preparation of velouté distinguish it as the correct answer in this context.