Which cocktail uses a dash of dry vermouth in its recipe?

The answer is B, as the Dry Manhattan specifically calls for a dash of dry vermouth as part of its recipe. This cocktail is a variation of the classic Manhattan, which typically uses sweet vermouth. The distinction lies in the type of vermouth used; a Dry Manhattan typically uses rye or bourbon whiskey and emphasizes the dry vermouth, resulting in a lighter and drier flavor profile.

The other options do not align with this characteristic. The Rob Roy, for instance, is made using sweet vermouth and is typically associated with Scotch whisky, whereas the Bourbon Manhattan generally employs sweet vermouth as well. The Perfect So-Co Manhattan is traditionally made with both sweet and dry vermouth in equal measures, rather than favoring dry vermouth alone. Therefore, the use of dry vermouth in a Dry Manhattan is what sets it apart and confirms its selection as the correct answer.