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 when it crosses the upper branch of the reference meridian. This is the standard definition of the sidereal day, as it provides a consistent and reliable reference point for measuring time. The other options are incorrect because they do not accurately describe the start of the sidereal day. Option A refers to a specific longitude, which is not the defining characteristic of the sidereal day. Option B refers to the sun's position, which is not the reference point for the sidereal day. Option C refers to a calendar date, which is not the defining event for the sidereal day.