The correct answer is D) Sidereal year. The tropical year, which is the period between successive vernal equinoxes, differs from the sidereal year, which is the period of the Earth's revolution around the Sun with respect to the fixed stars, by approximately 20 minutes. This is because the tropical year is slightly shorter than the sidereal year due to the precession of the Earth's axis. The other options are incorrect because the equinoctial year and the astronomical year are both synonymous with the tropical year, and the natural year is not a commonly used term in this context.