The sun is overhead at the tropic of Capricorn on

The Earth's axis is tilted at approximately 23.5 degrees relative to its orbital plane. As the Earth orbits the Sun, this tilt causes the Sun to appear directly overhead at different latitudes throughout the year.

The Tropic of Capricorn is located at latitude 23.5 degrees S. The Sun is directly overhead at this latitude during the December solstice, which occurs on or around December 22. At this time, the Southern Hemisphere is tilted toward the Sun, making it the longest day in the Southern Hemisphere and the shortest in the Northern Hemisphere.

The other dates correspond to different astronomical events:

• June 21 is the summer solstice, when the Sun is overhead at the Tropic of Cancer (23.5 degrees N).
• September 23 is the autumnal equinox, when the Sun is overhead at the Equator.
• March 21 is the vernal equinox, when the Sun is again overhead at the Equator.

The correct answer is December 22.