The correct answer is c. crenellation. The Colosseum has all three Greek orders (Doric, Ionic, and Corinthian) in its columns, it has many arches (think semicircular openings), and it has a post-and-lintel system (in which vertical elements -- like columns -- hold up horizontal elements, with large spaces between these two elements -- like arches). However, the Colosseum does not have crenellation, which is a battlement often seen on castles. A crenellation is a low protective wall on the roof of a building, with repetitive, squared openings from which people could shoot intruders.
