Final answer:
The term 'umbra' refers to the area where light is completely blocked, creating the darkest part of a shadow during events like eclipses. It contrasts with the penumbra, which is where the light source is only partially blocked.
Step-by-step explanation:
The most accurate definition of "umbra" is c. area where light is blocked. During an eclipse, the shadows cast by Earth or the Moon contain two main parts: the umbra and the penumbra. The umbra is the darkest part of the shadow, where the light source is completely blocked, resulting in the absence of direct light. In the context of a solar eclipse, if the Moon's umbra reaches Earth's surface, it can cause a total eclipse, enveloping the observer in darkness as it blocks the Sun.
For a lunar eclipse, it is only considered total if the Moon passes directly through Earth's umbra. This contrasts with the penumbra, which is a lighter shadow cast around the umbra where the light source is only partially blocked, leading to partial eclipses.