Which of the following is a primary ingredient in a Moscow Mule?

The Moscow Mule is a classic cocktail that is primarily known for its refreshing and zesty flavor profile, which comes from its core ingredient: vodka. This spirit provides a neutral base that allows the other components, such as ginger beer and lime juice, to shine through. The combination of vodka, spicy ginger beer, and tart lime juice served in a chilled copper mug has made the Moscow Mule a popular choice among cocktail enthusiasts. Vodka’s characteristic smoothness complements the other flavors, making it an essential ingredient in this cocktail.

Rum, gin, and whiskey do not fit the traditional recipe for a Moscow Mule, as they impart different flavor profiles and textures that are not in line with the drink's identity. Each of these spirits has its own unique characteristics that would fundamentally change the way a Moscow Mule tastes. The use of vodka is what makes this cocktail distinct and easily recognizable.