Which architectural feature was enabled by the discovery of concrete in Roman architecture?

The discovery of concrete was pivotal in the advancement of Roman architecture, enabling the widespread use of structural forms such as arches, domes, and vaults. Concrete is a versatile building material that can be molded into various shapes and provides significant compressive strength. This quality allowed Romans to construct larger and more complex structures than ever before.

Arches distribute weight more evenly than traditional post-and-lintel structures, allowing for wider openings and stronger supports. The use of concrete made it possible to build massive vaults and domes, exemplified by structures like the Pantheon, which features a large dome with a central oculus, showcasing the material’s capability to create expansive interior spaces without relying on heavy stone blocks.

The other choices present features that, while present in Roman architecture, do not specifically relate to the critical innovations brought about by concrete. For instance, columns with ornate designs can be built from stone or marble without needing concrete, and flat-roofed structures were achievable even before the advent of concrete. The mention of simple stone blocks indicates more traditional building techniques that predate the Roman innovation of concrete usage. Thus, the correct answer underscores the transformative impact of concrete on architectural forms in Roman times.