The correct answer is A) at lower transit. A latitude line is obtained by observing a celestial body at its lower transit, which is when the body crosses the observer's meridian. At this point, the body's altitude is at its minimum, and its azimuth is due north or south. By observing the body's altitude at lower transit, the observer can determine their latitude, as the altitude of the body at this point is equal to the observer's latitude. The other options are incorrect because observing a body on the celestial horizon, the prime vertical, or the Greenwich meridian would not provide the necessary information to determine the observer's latitude.