Final answer:
Flying buttresses were an architectural solution in Gothic architecture, not Romanesque. Romanesque buildings used thick walls and massive columns for support. The options provided refer to column styles, which were interior elements in Romanesque architecture.
Step-by-step explanation:
The architectural element used for the exterior supports on Romanesque buildings is not listed among the options provided. Romanesque architecture, popular from around 800 to 1100 CE, featured architectural elements like barrel or groin vault ceilings, thick walls with low exterior buttresses, and squared-off towers. However, flying buttresses were a feature that became prominent in the subsequent Gothic period. These elements served as exterior load-bearing columns that were connected to the main structure by a segmented arch, helping the buildings support their own weight. Romanesque columns, massive and made of ashlar masonry and rubble, were used as interior support elements, rather than exterior. Gothic architecture is known for its use of flying buttresses that provided the support necessary for taller, thinner walls with large stained glass windows, ushering in a new era of architectural design with elements such as pointed arches and ribbed vaults.