Which of the following cocktails contains Triple Sec?

The Cosmopolitan is a cocktail that prominently features Triple Sec, which is a type of orange-flavored liqueur. This ingredient is essential to its flavor profile, contributing a sweet and citrusy note that balances with the acidity of lime juice and the depth of vodka. The standard recipe for a Cosmopolitan typically includes vodka, cranberry juice, lime juice, and Triple Sec. The combination produces a refreshing drink that is both visually appealing and palatable.

In contrast, the Old Fashioned focuses on whiskey (typically bourbon or rye) with bitters, sugar, and a twist of citrus, making it a completely different category of cocktail that does not utilize Triple Sec. The Regular Rob Roy, a variation of the Manhattan made with Scotch whisky, also does not contain Triple Sec. Lastly, the Gimlet Martini, which usually consists of gin and lime juice (and sometimes a sweetener), does not typically feature Triple Sec either. This highlights the unique role Triple Sec plays within the Cosmopolitan, making it the correct answer.