The correct answer is A) zenith distance. The zenith distance is the angular distance of a celestial body from the zenith, which is the point directly overhead. In the given equation, 90° - Ho = zenith distance, where Ho represents the observed altitude of the celestial body. This relationship is fundamental in celestial navigation, as the zenith distance is used to calculate the position of the observer relative to the celestial body. The other options are incorrect because: B) co-latitude is the complement of the latitude, not the relationship between zenith distance and observed altitude. C) LHA (Local Hour Angle) is the angular distance of a celestial body east or west of the observer's meridian. D) sextant altitude is the observed altitude of the celestial body, not the difference between 90° and the observed altitude.