What are the minimum dimensions allowed for a flat?

The minimum dimensions for a flat are specified in postal regulations to ensure proper processing and handling through the postal system. The correct option provides dimensions that align with the requirements set by the USPS for flat mail.

For flat mail, the established minimum height, length, and thickness contribute to its eligibility for being classified as a flat. The dimensions given in the correct option—6 1/8 inches in height, 11 1/2 inches in length, and 0.25 inches in thickness—meet the minimum size standards perfectly. This allows the piece to be processed effectively without issues that could arise from being outside of these specified limits.

The other options either fall short of these minimum requirements in at least one dimension. This is why option B stands out as the valid choice, as it adheres to the set criteria, enabling successful processing within the USPS system.