Answer: D keystone
Explanation:
The Keystone slightly protrudes from the wall at the point where the arch springs from- a pendentive is a curved triangle of vaulting formed by the intersection of a dome with its supporting arches , a Voussoir is a wedge-shaped or tapered stone used to construct an arch, and an impost ( the second closest to the definition ) is that part of an arch from which the arc swings upward.
