The correct answer is C) centers of the various celestial bodies. The values of the Greenwich hour angle and declination, as tabulated in almanacs, are for the centers of the various celestial bodies, not their upper or lower limbs. This is because the celestial coordinates (right ascension and declination) refer to the central point of the celestial body, not its edge. Measuring from the center provides a more precise and consistent reference point for navigation and astronomical calculations. The other options are incorrect because the upper or lower limbs of a body would not provide the same standardized reference point as the center. Navigational and astronomical data is based on the central positions of celestial objects.