Which cocktail is known for its signature ingredient, mint leaves?

The cocktail known for its signature ingredient of mint leaves is the Mojito. This refreshing drink is traditionally made with fresh mint leaves muddled with sugar and lime juice, combined with white rum and soda water. The mint adds a distinctive fresh flavor and aromatic quality, which is essential to the Mojito's identity.

In contrast, the Old Fashioned primarily features whiskey, sugar, and bitters, with a citrus twist but does not include mint. The Sazerac is characterized by rye whiskey or cognac, sugar, and absinthe, with no mint involved. The Negroni is made with gin, Campari, and sweet vermouth, and also does not include mint. Thus, the Mojito stands out distinctly for its use of mint, making it the correct answer.