What ingredient is typically used to make a béarnaise sauce?

Béarnaise sauce is a classic French sauce that is an emulsion of egg yolks and butter, flavored with the inclusion of aromatics. The key ingredient in béarnaise sauce is tarragon, which provides a distinctive anise-like flavor that characterizes the sauce. This herb is often accompanied by other ingredients such as shallots, white wine vinegar, and sometimes chervil, but tarragon is essential for achieving the sauce's signature taste.

In contrast, while basil, chives, and parsley are all herbs used in various culinary applications, they do not impart the same flavor profile as tarragon in the context of béarnaise sauce. Each of these herbs is suitable for different sauces and dishes, but they do not belong to the traditional composition of béarnaise, which specifically highlights tarragon as its defining flavoring agent. Therefore, tarragon is the correct choice for making béarnaise sauce.