Which of the following specifications must be met to classify a spirit as bourbon?

To classify a spirit as bourbon, one of the essential criteria is that it must be distilled in the United States and contain at least 51% corn in its mash bill. This requirement is foundational to bourbon's identity, as it directly influences the flavor profile and character of the spirit. The use of corn as the principal grain contributes sweetness and a smooth finish, which are hallmark characteristics of bourbon.

The other specifications listed are not necessary for a spirit to be classified as bourbon. For instance, while aging bourbon for at least two years is common to develop its flavors, there is no requirement for it to be aged for a minimum of five years. Furthermore, the inclusion of rye is not mandatory; though rye can be part of a bourbon's mash, it's not a requisite as long as corn remains over 51%. Lastly, while bourbon can certainly be bottled at 70 proof (which is 35% ABV), it is not a requirement for being classified as bourbon, as it must be bottled at a minimum of 80 proof (40% ABV). Thus, option B accurately captures the primary regulations governing bourbon classification.