Final answer:
The solstices and equinoxes mark changes in Earth's axial tilt and its orbit around the Sun, which is answer option B. They are key to the progression of seasons and are rooted in the fundamental motions of the Earth in relation to the Sun.
Step-by-step explanation:
The solstices and equinoxes are significant astronomical events that mark the changes in Earth's axial tilt and its orbit around the Sun. This directly relates to the change of seasons we experience. The correct answer to the question is B) Earth's axial tilt and its orbit around the Sun. During the summer solstice, the Sun is higher in the sky and its rays strike Earth more directly, leading to longer days and higher temperatures. Conversely, the winter solstice has the Sun low in the sky with shorter days, resulting in cooler temperatures. Equinoxes occur twice each year when the Earth's axis is not tilted toward or away from the Sun, resulting in nearly equal day and night lengths. Specifically, the vernal equinox happens around March 21 and the autumnal equinox around September 22. These astronomical phenomena are integral to understanding the seasons and timekeeping on Earth.