The correct answer is A) Index mirror and horizon glass not being parallel. The index error in a sextant is primarily caused by the index mirror and horizon glass not being parallel. When these two components are not parallel, it creates an offset in the measurements taken with the sextant, which is known as the index error. This index error must be accounted for when taking sextant readings to ensure accurate celestial navigation. The other options, B, C, and D, are not the principal cause of index error. While issues with the telescope alignment or the perpendicularity of the mirrors can contribute to other sextant errors, they do not directly cause the index error in the same way as the lack of parallelism between the index mirror and horizon glass.