Which of these is a mixture of fat and flour used in making sauces?

Roux is flour and fat cooked together and used to thicken sauces. Roux is typically made from equal parts of flour and fat by weight. The flour is added to the melted fat or oil on the stove top, blended until smooth, and cooked to the desired level of brownness. Butter, bacon drippings or lard are commonly used fats. Roux is used as a thickening agent for gravy, sauces, soups and stews. It provides the base for a dish, and other ingredients are added after the roux is complete.

The fat is most often butter in French cuisine, but may be lard or vegetable oil in other cuisines. The roux is used in three of the five mother sauces of classical French cooking: béchamel sauce, velouté sauce, and espagnole sauce.

In Cajun cuisine, roux is made with bacon fat or oil instead of butter and cooked to a medium or dark brown color, which lends much richness of flavor, but makes it thinner.

Central European cuisine often uses lard (in its rendered form) or more recently vegetable oil instead of butter for the preparation of roux, which is called zápražka in Slovak, jíška in Czech, zasmażka in Polish, zaprška (запршка) in Bosnian, Croatian, Serbian, and Macedonian, zaprazhka in Bulgarian, rántás in Hungarian and Mehlschwitze in German.

Japanese curry, or karē (カレー), is made from a roux made by frying yellow curry powder, butter or oil, and flour together. The French term roux has become a loanword in Japanese, rū (ルー), or more specifically karērū (カレールー, curry roux).