Which of the following is an example of a sour cocktail?

A Pisco Sour is indeed an example of a sour cocktail. The term "sour" in cocktails typically indicates a drink that balances a base spirit with a souring agent, usually citrus juice, and often includes sugar or a sweetener to achieve a harmonious taste.

In the case of a Pisco Sour, it combines pisco, which is a grape brandy from Peru or Chile, with fresh lime juice as the sour component, and simple syrup to add sweetness. This balance of flavors—citrus, spirit, and sweetness—is what characterizes it as a sour cocktail.

When considering other options, a Martini is a classic cocktail primarily made of gin and vermouth, which doesn’t include a sour element. The Old Fashioned is a whiskey-based cocktail that relies on sugar and bitters, and it lacks the acidity that defines sour cocktails. Similarly, a Negroni features gin, vermouth, and Campari, offering bitter and sweet flavors without the essential sour component. Therefore, the Pisco Sour stands out as the true example of a sour cocktail among the choices provided.