An astronomical unit (a. u.) is
the diameter of the sun
the distance from the sun to the edge of the solar system
the distance from the sun to the earth
the mass of the earth

The Astronomical Unit is the distance from Earth to the Sun - the correct answer is the third one.
We use it, mostly with the reference to distances in the Solar System because we have an intuitive understanding of how big this distance is, and we can use it as a useful reference. 

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