Answer:

Introduction:
An Astronomical Unit (AU) is a unit of measurement used in astronomy to represent the average distance between the Earth and the Sun. This unit is widely used to describe distances within our solar system.

Explanation:
The correct answer is option A, which states that an Astronomical Unit is the average distance between Earth and the Sun. Let's understand this in detail:

What is an Astronomical Unit (AU)?
- An Astronomical Unit (AU) is defined as the average distance between the Earth and the Sun.
- It is approximately equal to 149.6 million kilometers (93 million miles).
- AU is used as a convenient unit of measurement to describe distances within our solar system.

Why is Earth and Sun the correct answer?
- The average distance between the Earth and the Sun is approximately 149.6 million kilometers or 93 million miles.
- This distance is considered as one Astronomical Unit (AU).
- The Earth orbits around the Sun in an elliptical path, and the average distance between the two is taken as the AU.
- The AU is used as a standard reference point for measuring distances within the solar system.

Why are the other options incorrect?
b) Earth and the Moon:
- The average distance between the Earth and the Moon is about 384,400 kilometers (238,900 miles).
- This distance is much smaller than the AU.
- The Moon is Earth's natural satellite and orbits around the Earth, not the Sun.

c) Jupiter and the Sun:
- Jupiter is the largest planet in our solar system and it orbits around the Sun.
- However, the average distance between Jupiter and the Sun is not equal to one AU.
- Jupiter's average distance from the Sun is approximately 778.5 million kilometers (484 million miles).

d) Pluto and the Sun:
- Pluto used to be considered the ninth planet in our solar system but was reclassified as a dwarf planet in 2006.
- The average distance between Pluto and the Sun is about 5.9 billion kilometers (3.67 billion miles).
- This distance is much larger than one AU.

Conclusion:
In conclusion, an Astronomical Unit (AU) is the average distance between the Earth and the Sun. It is used as a unit of measurement to describe distances within our solar system. The average distance between the Earth and the Sun is approximately 149.6 million kilometers (93 million miles), which is equal to one AU. The other options, Earth and the Moon, Jupiter and the Sun, and Pluto and the Sun, have different distances that do not match the definition of an AU.