The sauce made from ketchup and mayonnaise commonly served with fries in Utah is fry sauce.
Fry sauce is a simple pink condiment whose basic ingredients are tomato ketchup and mayonnaise. Some recipes add mustard, pickle relish, vinegar, garlic, or spices, but those variations do not change its defining identity. In Utah, it is especially associated with French fries and fast-food restaurants.
The sauce was popularized in Utah by Don Carlos Edwards, who served a pink sauce at his Don Carlos Barbecue restaurant sometime between 1941 and 1943. Edwards later served it at the first Arctic Circle restaurant in Salt Lake City, which opened in 1950. Arctic Circle helped spread the condiment across the western United States.
A common mix-up is treating fry sauce as simply another name for burger sauce. The terms can overlap in some countries, but burger sauce often includes chopped pickles or additional seasonings. Similar ketchup-and-mayonnaise mixtures also exist internationally, including salsa golf in Argentina and Uruguay and mayokétchup in Puerto Rico. Utah’s regional association is what makes “fry sauce” the expected answer here.