An eclipse occurs when:

• A. When the Moon passes directly between the Sun and Earth
• B. When Earth is between the Moon and the Sun
• C. When the Moon is full
• D. When either the Moon or the Earth falls into the shadow of the other

Answer: (D)

Eclipses happen when Sun, Earth, and Moon line up so that one body enters the shadow of another. This can happen in two ways: the Moon moves into Earth's shadow (a lunar eclipse) or Earth moves into the Moon’s shadow (a solar eclipse). Saying that either the Moon or the Earth falls into the shadow of the other covers both of these possibilities, so it’s the best description.

The other ideas describe only one specific scenario or a condition that isn’t enough on its own: a full Moon isn’t guaranteed to produce an eclipse, and a Moon directly between the Sun and Earth describes a solar eclipse but doesn’t include the lunar-eclipse case.