The correct answer is: A. One less than mean solar days.
A tropical year is the time it takes for the Earth to orbit the Sun once relative to the vernal equinox. It is approximately 365.2422 mean solar days long. A mean solar day is the time it takes for the Earth to rotate on its axis once relative to the Sun. It is approximately 24 hours long.
The Earth’s orbit around the Sun is not a perfect circle. It is slightly elliptical, which means that the Earth’s distance from the Sun varies throughout the year. This variation in distance causes the Earth’s orbital speed to vary. The Earth is fastest when it is closest to the Sun (perihelion) and slowest when it is farthest from the Sun (aphelion).
The Earth’s rotation on its axis is also not constant. The Earth’s rotation is slowing down due to the tidal friction caused by the Moon’s gravity. This slowing of the Earth’s rotation causes the length of a mean solar day to increase over time.
The combination of the Earth’s elliptical orbit and the Earth’s slowing rotation causes the number of sidereal days in a tropical year to be one less than the number of mean solar days.
Option B is incorrect because the number of sidereal days in a tropical year is one less than the number of mean solar days.
Option C is incorrect because the number of sidereal days in a tropical year is not equal to the number of mean solar days.
Option D is incorrect because the number of sidereal days in a tropical year is not none of these.