During which phase of the moon may a solar eclipse occur?

New moon

Explanation:

A solar eclipse can only take place at the phase of new moon, when the moon passes directly between the sun and Earth and its shadows fall upon Earth's surface.

A solar eclipse happens when the moon is perfectly aligned between the Earth and the Sun, thus obscuring it.

If the moon is between the Earth and the Sun, the far side of the moon is lighted, and thus you have a new moon.