Which ingredient is included in a Dirty Martini?

A Dirty Martini is specifically characterized by the inclusion of olive juice, which adds a savory and briny flavor to the cocktail. This ingredient distinguishes it from a standard Martini, which typically consists of just gin and dry vermouth. The addition of olive juice transforms the drink and provides that signature "dirty" element.

Dry vermouth, while part of a classic Martini, is not a defining feature of a Dirty Martini. Chocolate syrup would not be used in any traditional preparation of a martini, and sweet vermouth is typically associated with cocktails like the Manhattan or Negroni, rather than the martini family. Therefore, the inclusion of a splash of olive juice is essential for a Dirty Martini, making it the correct answer.