Which cocktail is traditionally served in a coupe glass?

The cocktail that is traditionally served in a coupe glass is the martini. The coupe glass, with its distinctive wide, shallow bowl and stem, is often associated with classic cocktails that are served "up," meaning chilled and without ice. The design of the glass allows for aromatic cocktails like the martini to be enjoyed while also highlighting their visual appeal.

The martini itself has a long-standing history and has become a symbol of sophistication in cocktail culture. The shape of the coupe enhances the drinking experience by being easy to handle and allowing the aromas to be concentrated around the open rim. Thus, the martini's characteristics align perfectly with the traditional use of the coupe glass, making it the correct choice.