What is a common base ingredient in a whiskey sour?

The foundational ingredient in a whiskey sour is whiskey, which is essential to the drink's identity and flavor profile. A whiskey sour typically combines whiskey with lemon juice and a sweetener, such as simple syrup. The whiskey provides the distinct character and richness that balances the acidity of the lemon juice and the sweetness, creating a classic cocktail enjoyed for its refreshing yet bold taste.

The other choices, while popular spirits, do not serve as the base for a whiskey sour. Gin, vodka, and rum serve as key ingredients in different cocktails but do not align with the traditional preparation and essence of a whiskey sour. The drink fundamentally relies on whiskey, making it the correct choice for this question.