The most likely base for Sally's favorite dish of grilled chicken with a creamy mushroom sauce is Velouté.
Velouté is a light, savory sauce made with a white roux and a clear stock, typically chicken, veal, or fish. It is a versatile sauce that can be used as the base for a variety of dishes, including soups, stews, and sauces. The creamy mushroom sauce in Sally's favorite dish is likely made with a velouté base, as the velouté would provide the sauce with its smooth, rich texture and savory flavor.
The other four mother sauces are:
Béchamel: A white sauce made with a white roux and milk or cream.
Espagnole: A brown sauce made with a brown roux and brown stock.
Hollandaise: An emulsion of egg yolks, butter, and lemon juice.
Tomate: A tomato sauce made with tomatoes, onions, and herbs.
Of these four sauces, only béchamel is similar to velouté in terms of its color and texture. However, béchamel is typically used for dishes that have a more delicate flavor profile, such as fish or vegetables. The creamy mushroom sauce in Sally's favorite dish is likely to have a more robust flavor, which is why velouté is a more likely base.
Therefore, the most likely base for Sally's favorite dish of grilled chicken with a creamy mushroom sauce is velouté.