What protein is used for the "Steak Frites"?

The dish "Steak Frites" is traditionally characterized by its use of grilled steak, which is a classic protein choice. This pairing of steak and fries originates from French cuisine and highlights the flavors and textures of a well-cooked piece of beef complemented by crispy, golden French fries.

Grilled steak provides a rich, savory flavor that is enhanced by the grilling process, offering a charred exterior and juicy interior. This main component of the dish is what defines "Steak Frites," setting it apart from other protein options available.

Other proteins listed, such as pork tenderloin, grilled chicken, and salmon, do not align with the traditional description of "Steak Frites," as they represent completely different culinary profiles and would not deliver the same flavors or experience associated with this classic dish. Hence, grilled steak is unequivocally the correct choice for "Steak Frites."