One Astronomical Unit is the average distance between

An Astronomical Unit (AU) is defined as the average (mean) distance between the Earth and the Sun. Standard educational material and textbooks state that this mean Earth–Sun distance is about 150 million kilometres and is used as the basic distance unit within the Solar System [1]. More precise scientific sources specify the AU to high accuracy (IAU value ~149,597,871 km), but conceptually it remains the Earth–Sun separation and corresponds to the semimajor axis of Earth’s orbit; astronomers use it to express distances of planets (for example, Earth = 1 AU) and other Solar System objects. Hence option 1 is correct.

Sources

1. [1] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 2: The Solar System > 2.4. Planets > p. 25