Answer: A. Equinox.
Step-by-step explanation:
The correct answer is A. Equinox.
Equinox refers to the day(s) of the year when the Sun rises most directly in the East and sets most directly in the West. During an equinox, the length of day and night is approximately equal all over the world. The word "equinox" comes from the Latin words "aequus" meaning equal, and "nox" meaning night.
There are two equinoxes in a year: the vernal (spring) equinox and the autumnal equinox. The vernal equinox occurs around March 20th in the Northern Hemisphere and September 22nd in the Southern Hemisphere. On these days, the Sun crosses the celestial equator, and day and night are of approximately equal duration.
During the vernal equinox, the Northern Hemisphere starts to tilt toward the Sun, marking the beginning of spring. Conversely, during the autumnal equinox, the Northern Hemisphere starts to tilt away from the Sun, marking the beginning of autumn.
It's important to note that solstice (option B) refers to the day(s) of the year with the longest and shortest periods of daylight, while aphelion (option C) and perihelion (option D) refer to the points in Earth's orbit around the Sun when it is farthest and closest, respectively. These are distinct events from the equinox.