Which strongbox was commonly found in a Roman atrium?

The strongbox commonly found in a Roman atrium is the Arca. This term refers to a type of chest or strongbox used for storing valuables and documents, often placed in merchant's homes and public buildings. The atrium of a Roman house served as a central space where guests were received and where important family and household items, including the Arca, would be kept for security and convenience. The design and functionality of an Arca demonstrate the Roman emphasis on protection of wealth and important items, reflective of their societal values.

In contrast, Exedra refers to a semicircular recess or room, often used for gatherings or discussion, while Posticum denotes a rear entrance or back passage in a Roman house. Caupona is associated with an inn or tavern, serving food and drink to travelers. None of these terms refer specifically to strongboxes, which is why the Arca is the correct answer for this question.