The correct answer is A) circle of equal altitude. A line of position from a celestial observation is a segment of a circle of equal altitude, which is a circle on the celestial sphere that passes through all points having the same altitude (angular height) above the horizon. This is because the position of a celestial body can be determined by its altitude and azimuth, and a line of position represents all the possible locations where the celestial body could be at the time of observation. The other options are incorrect because a parallel of altitude is a line of constant altitude, a vertical circle is a great circle passing through the zenith and nadir, and a parallel of declination is a circle on the celestial sphere with a constant declination.