The error in a sextant altitude caused by refraction is greatest when the celestial body is __________.

Correct Answer: Option D — near the horizon

Atmospheric density and path length increase significantly near the horizon, maximizing refraction errors. As outlined in Bowditch, refraction is zero at the zenith and increases rapidly below 15 degrees altitude, reaching roughly 34 arcminutes directly on the horizon, requiring mandatory tabular corrections before sight reduction.