The correct answer is (a).
Equinox is the two periods in the year when day and night are equal. It occurs on 21st March and 22nd September.
On the equinox, the Sun crosses the equator from south to north in the Northern Hemisphere and from north to south in the Southern Hemisphere. This means that the Northern and Southern Hemispheres receive equal amounts of sunlight.
The equinox is a significant astronomical event because it marks the beginning of spring in the Northern Hemisphere and autumn in the Southern Hemisphere. It is also a time of celebration for many cultures around the world.
The other options are incorrect because they do not correspond to the dates of the equinox.