The correct answer is: A. March 21
The declination of the Sun is the angle between the Sun’s rays and the Earth’s equatorial plane. The Sun’s declination is always between -23.5° and 23.5°. On March 21, the Sun’s declination is 0°, which means that the Sun is directly overhead at the equator. This is the vernal equinox, which marks the beginning of spring in the Northern Hemisphere and autumn in the Southern Hemisphere.
The right ascension of the Sun is the angle between the Sun’s position and the vernal equinox, measured eastward along the celestial equator. The right ascension of the Sun is always between 0h and 24h. On March 21, the Sun’s right ascension is 12h, which means that the Sun is at its highest point in the sky at noon.
Option B is incorrect because June 21 is the summer solstice, when the Sun’s declination is at its maximum value of 23.5° N.
Option C is incorrect because September 21 is the autumnal equinox, when the Sun’s declination is 0°.
Option D is incorrect because December 21 is the winter solstice, when the Sun’s declination is at its minimum value of 23.5° S.