Which item must be rectangular and cannot exceed a thickness of .75 inches?

The correct response highlights the specific physical characteristics required for certain mailing categories as defined by postal regulations. In this case, the item classified as "flat" is designed to adhere to a rectangular shape and must maintain a maximum thickness of 0.75 inches.

Flats are typically used for larger envelopes or packages that do not meet the criteria to be categorized as parcels but are too thick to qualify as letters. This classification ensures that these items can be processed efficiently and without complications during the mailing and delivery process. By maintaining a rectangular shape within the specified thickness, the item is more likely to fit within the automated sorting systems used by postal services, ensuring smoother handling and distribution.

In contrast, parcels, letters, and postcards have different dimensional requirements or flexibility in their shapes and sizes, which makes them distinct from the specifications set for flats. This distinction is important for adherence to postal guidelines and helps customers ensure that their mail meets the necessary criteria for efficient processing.