When the moon completely covers the sun, it is known as an eclipse. During this phenomenon, there are three parts of the shadow that the Moon casts on Earth: the penumbra, the umbra, and the antumbra.

The penumbra is the outer part of the shadow and it creates a partial eclipse. In this case, the Moon only partially covers the Sun, resulting in a decrease in brightness and a subtle darkening of the sky.

The umbra is the dark, inner part of the shadow. When the Moon completely covers the Sun, it creates a total solar eclipse. In this scenario, the Sun is completely blocked, and observers within the umbra experience a temporary period of darkness.

The antumbra is the part of the shadow where the Moon appears smaller than the Sun. This occurs during an annular solar eclipse when the Moon is farther away from Earth, causing the Moon to not completely cover the Sun.

Therefore, the correct answer is option 2: the Umbra.