What is the primary reason for the changing seasons on Earth?

The primary reason for the changing seasons on Earth is the Earth's orbital tilt. The axial tilt of approximately 23.5 degrees is responsible for the variation in the angle and intensity of sunlight that different parts of the Earth receive throughout the year. As the Earth orbits the Sun, this tilt causes different hemispheres to receive sunlight more directly at certain times, leading to warmer temperatures (summer) while the opposite hemisphere receives sunlight at a more oblique angle, resulting in cooler temperatures (winter).

During the solstices, one hemisphere is tilted toward the Sun, experiencing longer days and more direct sunlight, while the other is tilted away, experiencing shorter days and less direct sunlight. This interplay of sunlight and angle leads to the seasonal cycle that characterizes life on Earth. The other options, while related to various aspects of Earth's relationship with the Sun, do not play as central a role in creating the seasonal changes as the axial tilt does.