Which of the following substances in cooking can add a nutty flavor when heated?

The substance that adds a nutty flavor when heated is roux. A roux is a mixture of flour and fat, typically butter or oil, that is cooked together to form a thickening agent for sauces and soups. When roux is cooked, the flour undergoes a process called browning or toasting. This cooking process not only develops a deeper color but also enhances the flavor, resulting in a rich, nutty taste due to the Maillard reaction, where amino acids and sugars react under heat.

In contrast, while butter and oil can develop flavors when heated, they do not create the same toasty, nutty flavor profile associated with roux. Salt serves to enhance flavors but does not contribute a nutty flavor itself. Therefore, the unique properties of roux make it the correct answer for providing that characteristic nutty taste when heated in cooking.