Which cocktail includes a float of dark rum?

A Rum Runner is a cocktail that typically features a combination of light rum, dark rum, banana liqueur, apricot brandy, orange juice, and grenadine. The characteristic element that sets this cocktail apart is the float of dark rum that is added on top, which not only enhances the flavor but also creates a visually appealing layer when served. The float contributes richness and complexity, balancing the sweetness of the other ingredients.

In contrast, the other options listed do not traditionally include a float of dark rum. A Grasshopper is a creamy, mint-flavored cocktail made with crème de menthe and crème de cacao. Sex on the Beach is a fruity mix primarily consisting of vodka, peach schnapps, cranberry juice, and orange juice. Lastly, a Margarita is a classic tequila-based cocktail with lime juice, orange liqueur, and is often served with salt on the rim of the glass. None of these cocktails utilize dark rum in a floating manner as a distinguishing feature, making Rum Runner the only correct choice that fits the description.