The correct answer is: C. both (a) and (b)
The umbra is the inner, darker part of a shadow. It is formed when an opaque object blocks all direct light from a light source. In the case of a lunar eclipse, the umbra is the part of the Earth that is completely covered by the Moon’s shadow.
The umbra is surrounded by the penumbra, which is the lighter part of the shadow. The penumbra is formed when an opaque object blocks some, but not all, direct light from a light source. In the case of a lunar eclipse, the penumbra is the part of the Earth that is partially covered by the Moon’s shadow.
The umbra and penumbra are formed by tangents to the sun and moon. A tangent is a line that touches a curve at a single point. In the case of the umbra and penumbra, the tangents are the lines that touch the sun and moon at the points where the shadow is cast.
The umbra is the darkest part of the shadow because it is the only part of the shadow that is completely covered by the opaque object. The penumbra is lighter than the umbra because it is not completely covered by the opaque object.