What is added to a Kir cocktail to enhance its flavor?

A Kir cocktail is traditionally composed of dry white wine and crème de cassis, which is a sweet blackcurrant liqueur. Adding crème de cassis enhances the drink's flavor by introducing a rich, fruity sweetness that complements the wine. This balance creates a refreshing and aromatic cocktail that is both delicious and visually appealing, as the dark red liqueur mingles with the white wine.

The other options do not align with the traditional preparation or flavor profile of a Kir. Mint leaves might add an interesting element, but they are not a defining characteristic of the cocktail. Lemon juice and sour mix would alter the intended profile by introducing acidity that overshadows the delicate flavors of the white wine and blackcurrant liqueur. Therefore, crème de cassis is the essential ingredient that not only enhances the flavor but also maintains the integrity of the classic Kir cocktail.