Which architectural feature is used to support a circular dome over a square space in Byzantine buildings?

The correct choice is the use of pendentives in Byzantine architecture. Pendentives are triangular architectural elements that transition the support from a square base to a circular dome. In structures such as the Hagia Sophia, the use of pendentives allowed for the creation of a vast, open interior space topped with a dome, which is a hallmark of Byzantine design. The beauty of the dome is achieved while also maintaining the structural integrity of the building, as the weight of the dome is distributed down through the pendentives to the supporting elements below. This innovative technique not only serves a functional purpose but also has aesthetic implications, allowing for the grand scale and light-filled interiors that characterize Byzantine churches and structures.