The correct answer is C) zenith distance. The zenith distance is the angular distance between the celestial body and the observer's zenith (the point directly overhead). At the time of upper transit, the difference in latitude between the geographic position of the celestial body and the observer's position is represented by the zenith distance. The other options are incorrect because: A) Colatitude is the angular distance from the celestial body to the celestial pole, not the observer's position. B) Codistance is not a term used in this context. D) Altitude is the angular distance of the celestial body above the horizon, not the difference in latitude.