What is a nonadjustable error of the sextant?

Correct Answer: Option A — Prismatic error

Prismatic error is a nonadjustable error inherent in the manufacture of the sextant's mirrors or shades. Unlike index error, side error, or error of collimation, which can be corrected by the navigator using adjustment screws, prismatic error cannot be removed and must be accounted for via correction tables.