Which type of alcohol is used in a Moscow Mule?

The Moscow Mule is traditionally made with vodka, making it the correct choice for this question. This cocktail combines vodka with ginger beer and lime juice, served in a distinctive copper mug. The key ingredient of vodka is critical to the character of the drink; it provides a clean, neutral spirit that allows the flavors of the ginger beer and lime to shine through. Additionally, vodka's smoothness complements the spicy notes of the ginger beer, resulting in a refreshing beverage that highlights the unique blend of ingredients.

The other types of alcohol listed do not belong in a Moscow Mule. Gin, for instance, has a distinct botanical flavor that would alter the drink's intended profile significantly. Rum, with its sweet and diverse flavors, would not typically be used in this mix either. Whiskey, known for its robust and rich character, is also inappropriate for the classic Moscow Mule. Thus, vodka is the definitive spirit used in this popular cocktail.