The correct answer is D) Azimuth. In the horizon system of coordinates, the azimuth is equivalent to the local hour angle of the celestial equator system. Azimuth is the horizontal angular distance measured clockwise from the north point of the horizon to the vertical circle passing through the body. This is analogous to the local hour angle, which is the angular distance between the celestial meridian and the hour circle passing through the celestial body. The other options are incorrect because altitude and zenith distance are vertical coordinates, not horizontal, and colongitude is not a term used in the horizon system.