On March 21 each year, all towns in the southern hemisphere experience?

On March 21, all towns in the southern hemisphere (and indeed everywhere on Earth) experience equal length of day and night. This date marks the March equinox (also called the vernal equinox in the Northern Hemisphere or the autumnal equinox in the Southern Hemisphere).

During an equinox, the Sun is positioned directly over the Equator. This means the circle of illumination (the boundary between the sunlit and dark halves of the Earth) passes through both poles and bisects every line of latitude equally. As a result, every location on Earth receives approximately 12 hours of daylight and 12 hours of darkness.

The word "equinox" itself comes from the Latin aequus (equal) and nox (night), reflecting this equal division of day and night.

The other options are incorrect: there is no longer period of twilight specific to this date, the day is not 14 hours long (it is 12 hours), and there is no partial darkness during daytime. The September equinox (around September 23) produces the same effect.