Name a popular cocktail that uses vodka.

The Moscow Mule is a well-known cocktail that prominently features vodka as its base spirit. This refreshing drink consists of vodka, ginger beer, and lime juice, typically served in a copper mug, which enhances the experience by maintaining the drink's cold temperature. The combination of flavors—spicy from the ginger, tart from the lime, and the neutral profile of the vodka—makes it a favorite among cocktail enthusiasts.

In contrast, the other options either do not include vodka or are based on different spirits. For instance, a Gin and Tonic relies on gin, an Old Fashioned is traditionally made with whiskey, and a Tequila Sunrise features tequila. Each of these cocktails represents its own distinct category within the world of spirits, but they do not showcase vodka as their primary ingredient. Thus, the Moscow Mule stands out as the correct choice when identifying a popular vodka-based cocktail.