Which of the following is NOT one of the "Mother Sauces"?

The reason "Pesto" is the answer is that it does not fall under the category of the five classical "Mother Sauces" in French cuisine. The traditional Mother Sauces are foundational sauces from which a variety of other sauces can be derived. They include Béchamel, which is a white sauce made from milk and a white roux; Espagnole, or brown sauce, which is made from a brown stock and a brown roux; Hollandaise, a rich emulsion of egg yolk, butter, and lemon juice; and Tomato sauce, which is made from tomatoes and various seasonings.

Pesto, on the other hand, is a sauce that originated in Italy, made primarily from crushed basil, garlic, pine nuts, Parmesan cheese, and olive oil. It is a distinct sauce that does not serve as a base for other sauces in classical French cooking and therefore does not belong to the group of Mother Sauces. Understanding the distinction between these sauces is key in culinary arts, particularly when studying sauce preparation and classical cooking techniques.