The correct answer is D) It is difficult to establish the point where the sextant is vertical to the horizon. At high altitudes, it becomes challenging to accurately determine the point where the sextant is perfectly vertical to the horizon. This is because the horizon becomes less distinct and more distorted, making it harder to align the sextant precisely. This can lead to errors in the sun observation and, consequently, inaccuracies in the calculated position. The other options are incorrect because they do not directly address the primary issue with high-altitude sun observations. While rapidly changing altitudes, table inaccuracies, and refraction errors can be factors, the main problem is the difficulty in establishing the vertical alignment of the sextant to the horizon, which is crucial for obtaining accurate sun altitudes.