Which cocktail includes a cherry garnish and uses scotch as a primary ingredient?

The Rob Roy is a classic cocktail that prominently features Scotch whisky as its primary ingredient, setting it apart from the others listed. The drink is traditionally made with Scotch, sweet vermouth, and Angostura bitters, closely resembling the Manhattan, which uses rye or bourbon instead. A distinctive element of the Rob Roy is its garnish, typically consisting of a cherry. This garnish enhances the drink's presentation and adds a touch of flavor, aligning with the cocktail's overall character.

In contrast, the Manhattan utilizes whiskey, but not specifically Scotch, and is often garnished with either a cherry or a twist of lemon peel. The Sour Apple Martini focuses on apple flavors and does not incorporate Scotch at all, while the Cosmopolitan is known for its citrusy profile and uses vodka as its base spirit. Thus, the uniqueness of the Rob Roy lies in its incorporation of Scotch and its characteristic cherry garnish, making it the correct answer to the question.