The correct answer is - Spring.
The vernal equinox, also known as the spring equinox, marks the beginning of the spring. Its date varies, depending on the year, is it a leap year or not, though the variation is only a day from one year to another, so it can be either on the 20th or 21st of March.
The vernal equinox is not appearing at the same time in both hemispheres. While we do have equinox on both hemispheres, only on one of them is the vernal equinox, while on the other one is the autumn equinox. So we have a situation where in the Northern Hemisphere we have a vernal equinox, but on the Southern Hemisphere we have autumn equinox, and vice versa after approximately six months.
The equinox basically is the day in the year in which the day and the night have the same length.