One astronomical unit is the distance between the center of the Earth and the center of the ______
(i) Moon
(ii) Sun
(iii) Mars

Hint:Astronomical unit represents distance. If we know one astronomical unit then we can use that information to find the relative distance between any two bodies in the solar system.

Complete step by step answer:
Astronomical unit is represented by AU in short. Astronomical unit provides a convenient way to express and relate distance between bodies in the solar system and helps to carry out various astronomical calculations. The distance between Earth and Sun is taken as standard distance and it is called one astronomical unit. i.e. 1AU. The value of one astronomical unit, that is the distance between the Earth and the Sun is calculated by parallax method.
Hence the correct answer is (ii) Sun.
Additional Information:
Parallax is the apparent shift in the position of an object with respect to another when we switch from one eye to the other.
Working of parallax method: Close your right eye. Stretch your hand and extend your thumb out assuming it to be placed on some distant object. Now close your left eye without moving the thumb. You will observe that the position of your thumb has changed with respect to the distant object. By knowing the distance between your eyes and the angle that your eyes make with your thumb, you can calculate the distance between your eyes and your thumb. This same concept can be used to calculate the distance of any planet or star from earth.

Note: Earth is nearest to the sun in January and it is farthest from the Sun in July.
One astronomical unit (AU) = 90 million miles, which is 150 million kilometers (round figure).