The correct answer is C) Greenwich hour angle. The Greenwich hour angle (GHA) is the angle between the meridian of the observer and the meridian of Greenwich, measured westward along the celestial equator. This value, along with the declination of the celestial body, is used to determine the geographical position (GP) of the body for a high-altitude sight in celestial navigation. The other options are incorrect because: A) azimuth is the horizontal angle of a celestial body measured clockwise from north, B) right ascension is the celestial equivalent of longitude, and D) zenith distance is the angular distance of a celestial body from the observer's zenith.