Which of the following should be added to the sandwich only if requested?

The correct choice is cheese, as it is often considered an optional ingredient in many sandwich recipes. While some customers expect cheese on their sandwiches, it is not universally included and may be seen as an extra feature that can be added based on personal preference.

In the context of Dunkin Donuts and similar establishments, sandwiches may have a base recipe that includes specific core ingredients, while cheese often requires customer confirmation or request. Many customers may prefer their sandwiches without cheese due to dietary restrictions or personal taste.

Tomato, lettuce, and onion are generally more standard toppings that are typically included in most sandwich preparations unless specifically asked to omit them. This practice ensures that the customer receives the sandwich as they envision it, allowing for a more personalized experience.