What would you use to balance a whiskey in a Whiskey Sour?

In a Whiskey Sour, the primary function of balancing the whiskey is achieved through the use of sour mix. Sour mix is a combination of lemon or lime juice and simple syrup, which provides both acidity and sweetness. This balance is essential in creating a well-rounded cocktail, as the tartness from the citrus helps to cut through the richness of the whiskey, resulting in a refreshing drink.

While other ingredients, like sweet and sour mix, typically serve a similar purpose, "sour mix" specifically refers to the classic combination traditionally used in this cocktail, making it the most appropriate choice. Soda water can dilute the drink without enhancing flavor, cranberry juice introduces an entirely different taste profile, and sweet and sour mix, although similar, is a more generic blend that doesn't highlight the specific components called for in a Whiskey Sour. Thus, sour mix is the correct ingredient for balancing the whiskey in this classic drink.