The correct answer is D. All the above.
The true sun is the position of the sun in the sky as seen from Earth. The mean sun is a fictitious sun that moves across the sky at a constant rate, regardless of the Earth’s orbit around the sun. The true and mean suns occupy the same meridian at the same time on the vernal equinox (March 20 or 21), the summer solstice (June 20 or 21), the autumnal equinox (September 22 or 23), and the winter solstice (December 21 or 22).
On the vernal equinox, the sun crosses the equator from south to north. This marks the beginning of spring in the Northern Hemisphere and autumn in the Southern Hemisphere.
On the summer solstice, the sun reaches its highest point in the sky. This marks the beginning of summer in the Northern Hemisphere and winter in the Southern Hemisphere.
On the autumnal equinox, the sun crosses the equator from north to south. This marks the beginning of autumn in the Northern Hemisphere and spring in the Southern Hemisphere.
On the winter solstice, the sun reaches its lowest point in the sky. This marks the beginning of winter in the Northern Hemisphere and summer in the Southern Hemisphere.