Final answer:
The answer is C. Ribs, which are the arch-like structural members that support a vault at the groins in Gothic architecture, allowing for the construction of taller vaults by channeling weight downwards.
Step-by-step explanation:
The arch-like members that support a vault at the groins, defining its distinct surfaces or dividing these surfaces into panels, are known as C. Ribs. These ribs are a characteristic feature of Gothic architecture, providing both structural support and a decorative element to the vaulting system. In a typical Gothic ribbed vault, these ribs meet at the vault's apex and help to define the geometric pattern that is crucial to the vault's design and stability.
An important aspect of the ribbed structure is that it allowed builders to construct higher vaults than was possible with earlier, round arches. The ribbing channels the weight of the vault more directly downwards, rather than outwards, which is essential in the construction of tall and spacious interiors that Gothic architecture is famed for. While the term ribbed vaulting captures the essence of this structural innovation, other elements like keystone and coffers also play crucial roles in the broader category of vault construction.