What type of alcohol is used to float in a Rum Runner?

In a Rum Runner cocktail, the floating layer is typically achieved using Grenadine. This syrup has a vibrant red color and is thicker than most liquors, which helps it to float on top of the drink rather than mixing in. When Grenadine is poured gently over the back of a spoon or directly over the drink, it slowly descends, creating a beautiful visual effect as it sits atop the rum and other ingredients.

The other options do not serve this purpose effectively. Blue Curacao, while also a syrupy liqueur, does not have the same color contrast in the context of a Rum Runner, and its density is not sufficient for floating on top like Grenadine does. Kahlua, being a coffee liqueur, has a dark color and heavier profile that would mix more than float, and whiskey does not have the syrupy consistency needed for floating on cocktails. Thus, Grenadine is the ideal choice for achieving the intended presentation of a Rum Runner.