Answer:
C. when a portion of the moon passes through the earth's umbral shadow.
Step-by-step explanation:
A partial lunar eclipse occurs when the Earth passes between the sun and moon but the celestial bodies are not arranged in a straight line. When that occurs, a small part of the moon's surface is covered by the darkest, central part of the shadow of the Earth called the umbra. The rest is covered by the outer part of the shadow, termed penumbra.