Which unit is defined as the distance from the Earth to the Sun?

The correct answer is based on the definition of an astronomical unit (AU), which is specifically defined as the average distance from the Earth to the Sun. This unit of measurement is crucial in astronomy for expressing distances within our solar system, as it provides a convenient way to compare the distances of various celestial bodies relative to the distance to the Sun.

In contrast, a light year measures the distance light travels in one year, which is more applicable for interstellar distances rather than the more local distances within our solar system. A kilometer is a standard metric unit of length used for everyday measurements on Earth and is not suitable for astronomical distances. Finally, a parsec is a larger unit of distance used in astronomy to describe distances to stars and galaxies, equivalent to about 3.26 light years, making it inappropriate for the context of measuring the relatively shorter distance from the Earth to the Sun. Thus, the astronomical unit effectively captures this specific distance with a clear definition.