Which ingredient is part of a classic martini?

The classic martini is primarily composed of gin and dry vermouth, making the presence of dry vermouth crucial to this iconic cocktail. A dash of dry vermouth adds a subtle herbal flavor and complexity that complements the botanicals found in gin, creating a balanced and elegant drink. The use of dry vermouth is traditional and central to the martini's distinct profile, which has made it a staple in cocktail culture.

The other options do not fit the classic martini recipe. Sweet vermouth, for instance, is typically used in different cocktails, such as the Manhattan, and would markedly alter the taste profile of a martini. Orange bitters can be used in variations of a martini but are not a standard ingredient in the classic version. Grenadine, known for its sweetness and fruity profile, is generally associated with a different category of cocktails and is not compatible with the classic martini's taste.