The vernal equinox

happens during january
happens around march 21st
has days and nights of equal length
is the longest day of the year

The vernal equinox has days and nights of equal length.

Option C

Explanation:

Vernal equinox is the time of the year when the sun lies on equator of earth. This leads to the equal length of days and night all over the world. The vernal equinox on northern hemisphere lies around March 20 to 21 when there’s spring in northern hemisphere and Autumn in Southern hemisphere.

The vernal equinox is the time when there’s exact 12 hours of day and 12 hours of night.  After this, the Northern hemisphere approaches the summer solstice and southern hemisphere approaches the winter solstice.