When the index and horizon mirrors of a properly adjusted sextant are at an angle of 45° to each other, the arc reads __________.

Correct Answer: Option D — 90°

According to Bowditch, the sextant's design incorporates a double-reflection principle where the angle measured on the arc is half the true angle between the mirrors. Therefore, a 45-degree angle between the index and horizon mirrors doubles on the graduated arc to indicate 90 degrees.