The correct answer is C) Azimuth. In the horizon system of coordinates, the azimuth is equivalent to the Greenwich hour angle (GHA) of the celestial equator system. The azimuth is the horizontal angle measured clockwise from the north point of the horizon to the vertical circle passing through the object. Similarly, the GHA is the angle measured eastward along the celestial equator from the Greenwich meridian to the hour circle passing through the object. The other options are incorrect because they represent different coordinates in the horizon system, such as coaltitude (the complement of altitude) and zenith distance, which are not directly equivalent to the GHA.