Final answer:
The fall equinox in the Southern Hemisphere occurs on March 21, which is when day and night are approximately equal in length globally.
Step-by-step explanation:
The fall equinox in the Southern Hemisphere occurs around March 21, which is the correct answer, Option A. This is when the Earth's axis is not tilted toward or away from the Sun, resulting in approximately equal lengths of day and night across the planet.
It's the counterpart to the spring equinox in the Northern Hemisphere, which also happens around March 21. During the fall equinox, the Sun crosses the celestial equator moving southward, and it is a time when the length of day and night is nearly equal in all parts of the world.
This equinox marks the transition from summer to fall in the Southern Hemisphere and from winter to spring in the Northern Hemisphere.