The correct answer is A) the distance from a given point at the top of the sounding tube down to the surface of the liquid. A ullage reading is the measurement of the space or "vacant" volume in a tank, typically a cargo or fuel tank. This is measured from the top of the sounding tube, which extends down into the tank, to the surface of the liquid inside. This provides the distance from the top of the tank down to the current level of the liquid, which is the key information needed to determine the ullage or vacant volume in the tank. The other options are incorrect because they do not accurately describe what a ullage reading measures. Options B and C refer to the total depth of the tank, which is not the same as the ullage or vacant space. Option D refers to a measurement across the tank width, rather than the depth measurement that a ullage reading provides.