Which of the following elements is crucial to the stability of an arch?

The keystone is essential to the stability of an arch because it is the central stone at the apex of the arch that holds the structure together. The keystone is typically larger than the other stones and is shaped in such a way that it locks all the other stones in place. This design allows the weight of the arch to be distributed evenly downwards to the supports on either side, preventing the arch from collapsing. When force is applied to the arch, the keystone prevents the side stones from slipping out, maintaining the integrity and strength of the entire structure.

In contrast, while the outer stones, the base, and the side walls contribute to the overall strength and function of an arch, they rely on the keystone to provide the necessary stability that keeps the arch intact under load. This unique feature of the keystone emphasizes its critical role in arch construction and architectural design.