What is measured using a sextant's arm adjustment?

The correct answer reflects that the adjustment of a sextant’s arm is primarily used to measure the Index Error. The Index Error occurs when there is a discrepancy between the actual reading from the sextant and the true angle being measured. This error can happen due to misalignment in the optics or mechanical components of the sextant.

When the arm of the sextant is adjusted, it allows the user to bring the image of a celestial body down to the horizon or another reference point accurately. By making these adjustments and noting the readings, navigators can calculate the Index Error and correct their observations accordingly, ensuring more precise navigation calculations. The understanding and correction of Index Error are critical for accurate celestial navigation, as this directly affects the determination of a vessel’s position at sea.

The other potential options do not directly relate to what is specifically measured through the arm adjustment of a sextant.