The equinox

a. is when the subsolar point is at one of the tropics.
b. is the longest day of the year at any given place.
c. occurs four times during the year.
d. has 12 hours of day and 12 hours of night for all locations.

The equinoxes (from Latin aequinoctium (aequus nocte), "equal night") are the times of the year when the Sun is located in the plane of the celestial equator and comes from the Latin aequinoctium (aequus nocte), "equal night". It occurs twice a year: between March 19 and 21 and between September 21 and 24 of each year. As the name implies, on the dates on which the equinoxes occur, the day lasts approximately equal to that of the night in all places on Earth.

The equinoxes are used to set the beginning of spring and autumn in each hemisphere.

Therefore the correct answer is D.

the answer is a. at the equator