Which of the following ingredients is traditional in a classic Quiche Lorraine?

In a classic Quiche Lorraine, the traditional ingredient is bacon. This dish originates from the Lorraine region in France and is known for its rich and savory flavor profile. The key components of a traditional Quiche Lorraine include a pastry crust filled with a custard made of eggs and cream, and the addition of lardons, which are small strips or cubes of bacon that contribute a smoky, salty flavor which complements the creamy mixture beautifully.

While spinach, mushrooms, and tomatoes can certainly be found in various quiche recipes, they do not belong to the traditional formulation of Quiche Lorraine, which is primarily recognized for the inclusion of bacon. This distinct use of bacon is what sets it apart and defines its classic status in French cuisine.