Which of the following cannot be mailed as a flat?

When considering what can be mailed as a flat, it's important to recognize the specific criteria that define a flat item. Flats are typically defined as large envelopes or packages that meet certain dimensional limitations and are composed of flexible materials.

Books, unlike the other options, have a rigid structure and often exceed the thickness limits that are defined for flats. Due to their binding and cover, books are categorized as parcels rather than flats, which means they must be sent at a different rate and with different handling processes.

On the other hand, items such as photo albums, catalogs, and papers can generally be mailed as flats, provided they adhere to the weight and size requirements set forth by postal regulations. This distinction underlines the importance of material and construction when determining the mailing classification. Therefore, books are not eligible to be mailed as flats, confirming why this option is the correct choice.