The correct answer is: C. both (a) and (b)
The penumbra is the outer, lighter part of a shadow. It is formed when only part of an object is blocked by another object. In the case of the moon, the penumbra is formed when the moon passes through the outer part of the Earth’s shadow. The penumbra is not completely dark, because some sunlight is still able to reach it.
The umbra is the inner, darker part of a shadow. It is formed when an object is completely blocked by another object. In the case of the moon, the umbra is formed when the moon passes through the inner part of the Earth’s shadow. The umbra is completely dark, because no sunlight is able to reach it.
The penumbra and the umbra together form the shadow of the moon. The umbra is always smaller than the penumbra, because the Earth’s shadow is not perfectly round.
When the moon passes through the Earth’s shadow, we see a lunar eclipse. If the moon passes through the umbra, we see a total lunar eclipse. If the moon passes through the penumbra, we see a partial lunar eclipse.