Which ingredient is essential in a classic Daiquiri?

In a classic Daiquiri, the essential ingredient is rum. The Daiquiri is a cocktail that originates from Cuba and is traditionally made with three primary ingredients: rum, freshly squeezed lime juice, and sugar or simple syrup. Rum provides the foundational flavor profile of this drink, contributing its sweetness and complexity, which complements the tartness of the lime juice.

The other options do not align with the classic recipe. Gin, tequila, and whiskey aren't used in a traditional Daiquiri. Each of these spirits has distinct flavors and characteristics that cater to different cocktail styles, but they do not fit within the iconic structure of a Daiquiri. The classic preparation, emphasizing rum, is what defines this cocktail and distinguishes it from others that may use different spirits.