By definition, it's the Earth.
Earth (by definition)
Mercury. At only 0.39 AU (57,900,000km) from the sun, it is the closest planet in our solar system to the sun.
Jupiter is roughly 5.2 AU from the sun.
The Earth. Why would they chose another?
Uranus is around 19 AU from the sun, theres no planet that is 1.9 AU from the sun. the closest to this is Mars, at 1.5 AU.
Venus is 0.723332 AU (semi-major axis) from the Sun.
It is the earth that is in orbit one astronomical unit (AU) from the sun. That's where the AU came from. One AU is the mean distance from the sun to the earth. A link can be found below.
One AU is the distance between Earth and the Sun. No planet is that big across so you can only be talking about distance from the sun and in that case Earth by definition has an astronomical unit.
Planet Saturn is 9.5 au's away from the sun. An au is astronomical unit used to measure distances from a planet to the sun.
No planet so far observed or detected is that far from the Sun. The furthest planet so far observed is Neptune at about 30 AU.
Saturn, Its mean distance is 9.53 AU, where one AU (Astronomical Unit) is the earth to sun distance.
Jupiter
neptune