The correct answer is A. March 21.
On March 21, the Sun crosses the equator from south to north, marking the beginning of spring in the Northern Hemisphere and autumn in the Southern Hemisphere. This is known as the vernal equinox. The Sun’s declination is 0° at the equator, and its right ascension is 0° at the vernal equinox.
On June 21, the Sun reaches its highest point in the sky, which is known as the summer solstice. The Sun’s declination is 23° 27′ N at the summer solstice.
On September 21, the Sun crosses the equator from north to south, marking the beginning of autumn in the Northern Hemisphere and spring in the Southern Hemisphere. This is known as the autumnal equinox. The Sun’s declination is 0° at the equator, and its right ascension is 180° at the autumnal equinox.
On December 21, the Sun reaches its lowest point in the sky, which is known as the winter solstice. The Sun’s declination is 23° 27′ S at the winter solstice.