The correct answer is C) observer's meridian. The azimuth angle of a celestial body is measured from the observer's meridian, which is the vertical circle passing through the observer's zenith and the celestial body. This reference point is used to determine the azimuth angle, as it represents the observer's perspective and orientation. The other options are incorrect because: A) Zenith distance is the angular distance of a celestial body from the observer's zenith, not the reference point for measuring azimuth. B) The body's meridian is the vertical circle passing through the celestial body's north and south points, not the observer's reference point. D) The Greenwich meridian is the reference point for determining longitude, not the azimuth angle.