Which of the following cocktails includes coffee liqueur?

The Black Russian is a classic cocktail that specifically includes coffee liqueur as one of its main ingredients. This drink is typically made with vodka and coffee liqueur, resulting in a rich flavor profile that combines the warmth of coffee with the smoothness of vodka. The inclusion of coffee liqueur not only adds depth to the drink but also creates a unique balance that appeals to those who enjoy the taste of coffee.

In contrast, the other cocktails listed do not utilize coffee liqueur. The Madras is a fruity cocktail composed of vodka, cranberry juice, and orange juice, while the Lemon Drop Martini is a sweet and tangy drink made with vodka, lemon juice, and sugar. The Vodka Martini, a classic, focuses solely on the combination of vodka and dry vermouth, lacking any coffee elements. Thus, the Black Russian stands out as the only cocktail that incorporates coffee liqueur.