What factor is primarily responsible for the changing phases of the Moon?

The correct answer, focusing on the orbits of the Moon and Earth, emphasizes that the changing phases of the Moon are a result of the relative positions and motions of the Earth, Moon, and Sun. As the Moon orbits around the Earth, different portions of its illuminated surface become visible from our vantage point on Earth. This visibility changes in a cyclical manner, which creates the various phases of the Moon, such as new moon, waxing crescent, first quarter, and so on.

While the rotation of the Earth and the rotation of the Moon contribute to our experience of day and night and affect how we perceive the Moon in the sky, they do not directly cause the phases of the Moon. Additionally, while sunlight angles do play a role in the illumination of the Moon, they are not the primary factor at work. Instead, it is the interplay of the orbits of the Earth and Moon that leads to the observed changes in the Moon's appearance throughout its cycle.