The correct answer is C) Azimuth angle. In the horizon system of coordinates, the azimuth angle is equivalent to the meridian angle in the celestial equator system. The azimuth angle is the horizontal angle measured clockwise from the north point of the horizon to the vertical circle passing through the celestial body. This corresponds to the meridian angle, which is the angle between the celestial meridian and the hour circle passing through the celestial body in the celestial equator system. The other options are incorrect because: A) Zenith distance is the angular distance of a celestial body from the zenith, not the equivalent of the meridian angle. B) Colatitude is the angular distance of a point on the Earth's surface from the North Pole, not related to the horizon system. D) Altitude is the angular distance of a celestial body above the horizon, not the equivalent of the meridian angle.