The correct answer is C) equation of time. The difference between local apparent time (LAT) and local mean time (LMT) is indicated by the equation of time. The equation of time accounts for the difference between the sun's actual position in the sky (LAT) and the sun's mean position (LMT) due to the elliptical orbit of the Earth and the tilt of the Earth's axis. This difference is expressed in the equation of time, which navigators use to convert between LAT and LMT. The other options are incorrect because they do not directly represent the difference between LAT and LMT. Option A) longitude in time units represents the time difference due to longitude, while option B) difference of longitude between the local and central meridian in time units is a related but distinct concept. Option D) zone description refers to the time zone, which is a separate consideration from the equation of time.