The correct answer is A) Prismatic error. Prismatic error is a nonadjustable error of the sextant. It is caused by the prismatic glass in the sextant not being perfectly parallel, which can introduce a consistent offset in the angle measurements. This error cannot be adjusted or corrected by the user, unlike other sextant errors such as index error or error of collimation, which can be minimized through calibration and adjustment. The other options are incorrect because side error and index error are both adjustable errors, while error of collimation is an error related to the telescope, not the sextant itself.