What is the only moon phrase in which a solar eclipse can occur?

Answer 1

The New Moon. During that phase the Moon is approximately between the Earth and the Sun. The Moon only shines by the sunlight that reflects from it; when the Moon is between the Earth and the Sun the side of the Moon that is in the light is the side away from us, therefore the side that's toward us is in the dark-- and that's what the New Moon is, the time when the Moon appears dark to us.

Eclipses don't happen with every New Moon because the Moon's orbit is in a plane that is tilted with respect to the plane of the Earth's orbit around the Sun. That means that for most New Moons the Moon, if you could see it when it was closest to eclipsing the Sun, would pass "above" or "below" it rather than directly across it. But at a couple of points along its orbit the Moon IS directly in the plane of the Earth's orbit around the sun. If this happens at the same time as the New Moon, you get a solar eclipse.

Answer 2

A solar eclipse can only occur during a new moon phase, when the moon is positioned between the Earth and the Sun, causing the moon to block the Sun's light from reaching certain areas on Earth.