When does a solar eclipse occur?

A solar eclipse occurs when the Moon moves directly between the Sun and Earth, blocking the Sun's light either partially or completely. This alignment of the Sun, Moon, and Earth is essential for the phenomenon to happen. During a total solar eclipse, the Moon entirely covers the Sun, casting a shadow on Earth, while a partial solar eclipse occurs when only a part of the Sun is obscured.

The other contexts presented—such as a meteor shower, crossing the asteroid belt, or the appearance of Halley's comet—do not involve the specific alignment of the Sun and Moon with Earth that causes a solar eclipse. Meteor showers occur when Earth passes through the debris left by comets, while the asteroid belt is a region between Mars and Jupiter that does not relate to solar eclipses. Halley's comet's appearance is an event based on its orbital period and does not cause any direct solar obscuration. Therefore, the only correct answer highlights the critical alignment that defines a solar eclipse.