The correct answer is B) Azimuth angle. In the horizon system of coordinates, the azimuth angle is equivalent to longitude on the Earth. The azimuth angle is the horizontal angle measured clockwise from the north point of the horizon to the vertical circle passing through the object. This is analogous to how longitude is measured on the Earth, where it is the east-west angular distance from a reference meridian. The other options are incorrect because they do not directly correspond to longitude in the horizon system. Altitude is the vertical angle, zenith distance is the complement of altitude, and the horizon itself is the circular line where the sky and Earth meet, rather than a coordinate.