What is the architectural term for a sculpture that symbolizes boundaries and is often placed at entrances?

The correct answer is the term "Herm," which refers to a specific type of sculpture that often depicts a bust on a rectangular pedestal, typically with a phallus included. Historically, Hermes, the Greek god, was associated with these sculptures as they were believed to ward off evil and protect the spaces they marked. Herms were strategically placed at entrances to homes, public buildings, and various boundaries, symbolizing protection and the separation of different areas.

The other terms mentioned do not pertain to boundary markers in the same way. An "Exedra" refers to a semicircular space, often used in classical architecture, typically serving as a gathering area or a niche for seating. An "Oecus" is a term for a large room, specifically in Roman houses, which served various purposes, including dining or entertainment, but does not symbolize a boundary. "Arca," on the other hand, translates to a chest or box, often used for storage, and lacks the connection to territorial markers that Herms embody. Therefore, the significance and function of the Herm distinctly aligns it with the role of boundary symbolism at entrances.