What is the primary mixer for a Cape Codder?

The primary mixer for a Cape Codder is cranberry juice. This cocktail is traditionally made by combining vodka and cranberry juice, resulting in a refreshing drink that highlights the tartness of the cranberries. The flavor profile of the Cape Codder benefits significantly from this mixer, as it not only lends a vibrant color but also balances the alcohol's strength with its sweet and tart flavors, making it a popular choice in bars and at casual gatherings.

In contrast, the other options do not complement the traditional ingredients of a Cape Codder. Pineapple juice, orange juice, and tonic water would create entirely different flavor combinations that are not characteristic of this particular cocktail. Each of these mixers has its own unique taste profile, which does not mesh with the scintillating tang of cranberry juice, the signature component that defines the drink.