The correct answer is D) When the first point of Aries is over the upper branch of the reference meridian. The sidereal day is defined as the time it takes for the first point of Aries (the vernal equinox) to return to the same position in the sky, which is approximately 23 hours, 56 minutes, and 4 seconds. This is different from a solar day, which is the time it takes for the sun to return to the same position in the sky. The sidereal day begins when the first point of Aries, which is the reference point for celestial coordinates, is over the upper branch of the reference meridian. The other options are incorrect because they do not correctly identify the reference point for the sidereal day, which is the first point of Aries, not the sun.