Concept:The day when day and night are equal all over the Earth is known as an equinox.Explanation:During an equinox, the Sun is positioned directly above the Equator.This makes the lengths of day and night almost equal at all latitudes.The two equinoxes occur around 21st March and 23rd September each year.Since 21st March is the March equinox, it matches the condition given in the question.Answer:C. 21st March