Which of the following is classified as a liqueur?

Frangelico is classified as a liqueur because it is a sweetened alcoholic beverage infused with flavors, particularly of hazelnuts, vanilla, and herbs. Liqueurs are typically lower in alcohol content than spirits and are used in cocktails or enjoyed as a digestif due to their rich and sweet profiles.

In comparison, Bushmills and Chivas Regal are both types of whiskey, with Bushmills being an Irish whiskey and Chivas Regal a blended Scotch whiskey. These are distilled spirits known for their higher alcohol content and do not fall under the liqueur category. Miller, on the other hand, is a brand of beer, which is quite different from liqueurs both in terms of production and characteristics. Thus, Frangelico stands out as the only true liqueur in this selection.