What is the primary reason for the phases of the Moon?

The primary reason for the phases of the Moon is the relative positions of the Earth, Moon, and Sun. As the Moon orbits the Earth, the angle between the Earth, Moon, and Sun changes. This change affects how much of the Moon's surface is illuminated by sunlight and visible from Earth.

When the Moon is positioned between the Earth and the Sun, we see the Moon as a new moon since the sunlit side is facing away from us. Conversely, when the Earth is between the Moon and the Sun, the moon appears fully illuminated, which we call a full moon. The intermediate phases, such as the first quarter and last quarter, occur as the Moon moves through its orbit and different portions of its illuminated side become visible from Earth.

The other choices, though related to astronomical phenomena, do not directly cause the Moon's phases. The tilt of the Earth's axis influences the seasons but not the Moon's phases. The rotation of the Earth affects our perspective of the Moon in the night sky but does not influence its phases. Finally, while the distance of the Moon from the Earth plays a role in its size and brightness, it does not affect the cycling through different illuminated states that creates the phases.