Which of the following statements is/are true about technology used in the…

In arcuate (true-arch) construction, wedge-shaped stones are laid in a curve so that their combined thrust carries the weight of the structure above an opening down along that curve into the piers or walls on either side, rather than relying on a single horizontal beam as in trabeate (post-and-lintel) construction. The stone set at the crown, at the very top of the curve, locks the two haunches of the arch together and is called the keystone.

Statement A restates this load-transfer function of the arch, so it matches the concept above. Statement B assigns the name "base stone" to the stone at the crown of the arch, but the accepted technical name for that crown stone is "keystone", not "base stone". Since Statement A matches the accepted description of the arch's function while Statement B misnames the crown stone, only Statement A is a true statement about twelfth-century arch technology.

This is consistent with the broader account of Delhi Sultanate-era building: the arcuate technique (arches and domes) is described as replacing the earlier trabeate (beam-and-lintel) method precisely because the curved arch could carry the load of the superstructure down its own curve to the supports, allowing larger, pillar-free interior spaces — the same load-transfer role Statement A describes.

• "Neither A nor B" would require the load-transfer description of the arch to be inapplicable as well, but carrying the superstructure's weight down the curve to the piers is exactly the documented function of the arch, so this combination does not fit.

• "Only B" would require the load-transfer description in Statement A to be inapplicable while the name "base stone" for the crown stone is applicable — but the crown stone's accepted name is "keystone", not "base stone", so this combination does not fit.

• "Both A and B" would require "base stone" to be the accepted name for the crown stone, but the accepted name is "keystone", so this combination does not fit either.

Hence only Statement A is true, and the correct choice is "Only A".