What are the minimum dimensions for flat-size mail?

The minimum dimensions for flat-size mail are indeed 6 ⅛ inches in height and 11 ½ inches in length with a thickness of ¼ inch. This configuration is specifically designed to meet the postal service requirements for items categorized as "flats."

Flats are essentially rectangular packages that are larger than letters but smaller than parcels. The specified minimum dimensions ensure that the mail can be processed efficiently and properly handled throughout the delivery system. Items that do not meet these dimensions are either classified differently or may incur additional postage, thus ensuring that the postal service can maintain standard processing guidelines for different types of mail.

In this particular set of choices, the other answers present dimensions that either fall below the required measurements for flat-size mail or exceed them. This makes option B the only suitable choice for meeting the standards laid out by the postal service for flat-size mail.