Final answer:
An AU, or Astronomical Unit, is the average distance from Earth to the Sun, about 149.6 million kilometers. The correct option is c Astronomical units; Earth and the Sun.
Step-by-step explanation:
The term AU stands for Astronomical Unit, which is a measure of distance within the solar system. Specifically, one AU represents the average distance between the Earth and the Sun. This average distance is important because the Earth's orbit is not a perfect circle, but an ellipse; therefore, the distance from the Earth to the Sun varies throughout the year.
The average of the closest point (perihelion) and the farthest point (aphelion) is taken to define the astronomical unit. This distance is approximately 149.6 million kilometers (or 1.496 × 10¹¹ meters), making it a convenient standard for expressing and comparing distances in the solar system. It's necessary to use the average distance to account for the varying distances that occur due to Earth's elliptical orbit.