Which of the following is an ingredient in a Rob Roy?

The Rob Roy is a classic cocktail that is essentially a variation of the Manhattan, but it uses Scotch whisky instead of bourbon or rye whiskey. One of the key ingredients in a Rob Roy is sweet vermouth, which adds depth and sweetness to the drink. The sweet vermouth balances the strong, robust flavors of the Scotch and enhances the overall complexity of the cocktail.

In a standard Rob Roy, the proportions typically include a mix of Scotch whisky and sweet vermouth, often garnished with a cherry or a lemon twist. By incorporating the sweet vermouth, the drink takes on a rich, aromatic profile that is essential to its character.

While vodka, rum, and bitters are ingredients that are used in various cocktails, they do not define the Rob Roy. Vodka is not traditionally part of this drink, nor is rum. Although bitters can be added to many cocktails for flavor nuances, they are not a defining ingredient in a Rob Roy; instead, the emphasis is placed on the balance of whisky and sweet vermouth. Therefore, sweet vermouth is a vital component of the Rob Roy, making it the correct choice.