Question

what is an astronomical unit?
the average distance between planets in the solar system
the average distance between the earth and the moon
the approximate diameter of the sun including its atmosphere
the average distance between the earth and the sun

Explanation:

An astronomical unit (AU) is a standard unit of measurement in astronomy, defined specifically as the average distance between Earth and the Sun, used to simplify measurements within our solar system.

Answer:

the average distance between the Earth and the Sun