Correct Answer: (a) 21 March and 23 September
Solution:Day and Night are of equal duration on the day of Equinox. Equinox literally means "equal day and equal night". An equinox is an astronomical event in which Sun is vertically overhead of the equator, which occurs twice each year, i.e., 20/21 March and 22/23rd September. Therefore, in the given option 23rd September is the correct answer.