In an Old Fashioned cocktail, which ingredient is used as a sweetener?

The Old Fashioned cocktail traditionally uses a sugar cube, specifically a brown sugar cube, as its sweetener. This choice aligns with the classic preparation of the drink, where the sugar is dissolved in a small amount of water or bitters before the whiskey is added. The brown sugar cube not only serves to sweeten the drink but also contributes a slight molasses flavor, enhancing the overall profile of the cocktail.

While honey, maple syrup, and agave nectar are all viable sweetening options for various cocktails and can add unique flavors, they deviate from the traditional recipe of the Old Fashioned. The use of a brown sugar cube is deeply rooted in the history of this cocktail, making it the correct choice in the context of the question.