Final answer:
A horizontal rib marking the crown of a vaulting compartment is known as a Ridge Rib. It is a key feature in Gothic architecture, contributing both to the stability and the decorative complexity of vaulted ceilings. (option A)
Step-by-step explanation:
The horizontal rib that marks the crown of a vaulting compartment is known as a Ridge Rib. It serves as a structural and decorative element in the complex geometry of a vault, particularly in Gothic architecture. The ribbed vaulting created in this style can consist of various ribs, including cross ribs like the tierceron, and on occasion, even more intricate elements such as lierne ribs to form complex web patterns or fan vaulting in later Gothic developments.
Gothic architecture is known for its emphasis on vertical lines and the ribbed vaulting technique which took advantage of the pointed ribbed groin vault for increased height and elegance. The ribs rise from thin columns, dividing into the characteristic Gothic arches and meeting at the crown of the vault.
The Ridge Rib is a pivotal feature in the rib vaulting system, representing the peak where various ribs align, thereby establishing the structural integrity and the aesthetic zenith of the vaulted ceiling.