Answer:
C. Beginning of autumn
Step-by-step explanation:
On this image we can see the position of the Earth toward the Sun in four different parts of the year. WE can see that in one part of the year the Northern Hemisphere is tilted toward the Sun, while in the opposite part of the year it is the Southern Hemisphere that is tilted toward the Sun. In between them, we have 21 March and 23 September when both hemispheres are equally exposed to the sunlight. These to dates mark the beginning of spring and beginning of autumn. As they are dates when both hemispheres receive equal amounts of sunlight, it means that these are the dates when the equinoxes appear on Earth.