What is the major problem with taking high altitude sun observations?

Correct Answer: Option D — It is difficult to establish the point where the sextant is vertical to the horizon.

At high altitudes, the celestial body is near the zenith. Establishing the vertical plane of the sextant becomes difficult because the horizon is distant and the arc of the circle is small, leading to significant errors in the measured altitude. This is a well-documented limitation in Bowditch.