The correct answer is C) Parallels of altitude. In the horizon system of coordinates, the parallels of altitude correspond to the parallels of declination in the celestial equator system. Parallels of altitude are the horizontal circles that are parallel to the horizon plane, just as parallels of declination are the circles parallel to the celestial equator in the celestial equator system. The other options are incorrect because: A) Azimuth angle is the horizontal angle measured from north, B) Vertical circles are great circles passing through the zenith and nadir, and D) Zenith distance is the angular distance from the zenith to a celestial body.