The correct answer is C) the time between the passage of the mean sun and the apparent sun over a meridian. The equation of time measures the difference between the time indicated by a sundial (local apparent time) and the time indicated by a clock (mean solar time). This difference is caused by the elliptical orbit of the Earth around the Sun and the tilt of the Earth's axis, which causes the apparent sun to move at a varying speed relative to the mean sun. By calculating this difference, navigators can convert between local apparent time and mean solar time, which is the standard time used for navigation. The other options are incorrect because they do not accurately describe what the equation of time measures. Option A refers to the difference between local apparent time and Greenwich apparent time, option B refers to longitude in time units, and option D refers to the difference between sidereal time and local time at the Greenwich meridian, none of which are the correct definition of the equation of time.