Which ingredient is part of a Negroni cocktail?

The Negroni is a classic cocktail that is traditionally composed of three main ingredients: gin, sweet vermouth, and Campari. Campari is a bitter Italian liqueur that is essential to the Negroni, providing its signature flavor profile which balances sweet and bitter notes. This ingredient plays a crucial role in defining the character of the cocktail, contributing to its complexity and depth.

Other ingredients listed, such as amaretto, grenadine, and pineapple juice, are not components of a traditional Negroni. Amaretto is a sweet almond-flavored liqueur that wouldn't complement the robust bitterness of the cocktail. Grenadine is a syrup commonly used in sweeter cocktails for color and sweetness, and pineapple juice adds a fruity element that does not align with the Negroni's intended flavor balance. Therefore, Campari is the correct answer as it's the core ingredient that makes the Negroni distinctive.