What is defined as any item pertaining to U.S. Government business that is mailed with prepaid postage?

The correct answer is that "Official Mail" is defined as any item pertaining to U.S. Government business that is mailed with prepaid postage. This category encompasses all forms of mail used by authorized personnel or agencies within the government to conduct business, ensuring that it is handled appropriately and in accordance with established regulations.

Official mail is often prioritized for efficiency and security, as it typically contains sensitive information or communication related to government operations. The distinction between official mail and other types of mail is crucial, as different regulations, handling procedures, and postage methods apply to non-official types of mail such as certified, military, or priority mail, which serve different purposes and audiences outside the context of government business.