Concept:The seasons in the Southern Hemisphere are opposite to those in the Northern Hemisphere.
Explanation:The autumnal equinox marks the beginning of autumn in a hemisphere.
In the Northern Hemisphere, the autumnal equinox occurs around 23rd September.
This is when the Sun crosses the equator moving southwards.
Meanwhile, the Southern Hemisphere begins its spring on that same date.
The Northern Hemisphere experiences its spring equinox around 21st March.
Because the Southern Hemisphere has opposite seasons, 21st March is the start of autumn there.
So 23rd September is actually the spring equinox in the Southern Hemisphere, not the autumn equinox.
Hence, the autumnal equinox in the Southern Hemisphere falls on 21st March.
Answer:A. 21st March