Abeka Science Earth and Space Test 8 Practice

An astronomical unit is a length used to describe distances in the solar system. It is defined by the average distance between Earth and the Sun, which reflects the size of Earth’s orbit. Because the orbit is a bit elliptical, the actual distance changes over the year, but the mean (average) distance is used as the standard. That average distance is about 149.6 million kilometers, often rounded to roughly 150 million km. This makes it a convenient reference for comparing how far other planets are from the Sun—for example, Mars sits at about 1.5 AU from the Sun.

The other options do not fit because the Moon is far closer (about 0.0026 AU, or ~384,000 km), and 5 million kilometers or 1 billion kilometers are not the defined average Earth–Sun distance.