Final answer:
Roman concrete was the development that allowed for cost-effective, large-scale construction, paired with their innovative use of arches.
Step-by-step explanation:
The Roman development that enabled them to cut costs and build on a massive scale was C. Concrete. By mastering the use of concrete, the Romans could construct larger and more complex structures more quickly and economically than using traditional stone. Coupled with their innovative use of the arch, which could bear immense weight by redistributing it from the top down into the ground, the Romans revolutionized architecture. These constructions were often covered with a façade of stucco, stone, or marble for aesthetic purposes. Concrete also facilitated the building of other significant structures such as aqueducts, which could span great distances thanks to arches and were lined with water-resistant cement.