The correct option is 21st March : Autumn
Explanation
Seasons are caused by the revolution of the Earth around the Sun and the tilt of the Earth's axis. The seasons in the Southern Hemisphere are always opposite to those in the Northern Hemisphere. When the Northern Hemisphere experiences summer, the Southern Hemisphere experiences winter, and vice versa.
Option Analysis
- Option (a) is Correct: On 21st March, the Sun is vertically overhead at the Equator. This position is known as the Equinox. During this time, it is Spring in the Northern Hemisphere and Autumn in the Southern Hemisphere.
- Option (b) is Incorrect: On 23rd September, the Sun is again vertically overhead at the Equator. This marks Autumn in the Northern Hemisphere and Spring in the Southern Hemisphere.
- Option (c) is Incorrect: On 21st June (Summer Solstice), the Sun is vertically overhead at the Tropic of Cancer. The Northern Hemisphere experiences Summer, while the Southern Hemisphere experiences Winter.
- Option (d) is Incorrect: On 22nd December (Winter Solstice), the Sun is vertically overhead at the Tropic of Capricorn. The Northern Hemisphere experiences Winter, while the Southern Hemisphere experiences Summer.
Key Takeaway: The cycle of seasons in the Southern Hemisphere is the inverse of the Northern Hemisphere; 21st March marks the Autumnal Equinox in the South, while 23rd September marks the Vernal (Spring) Equinox.