The correct answer is option-3: Astronomical Unit.
Option-1: Light Year is not the correct answer because a light year is defined as the distance light travels in one year, which is approximately 9.46 trillion kilometers. It is not equivalent to the mean distance between the Sun and Earth.
Option-2: Parallactic Second is not the correct answer. It is a unit of angular measurement used in astronomy to describe the apparent motion of a star due to parallax. It is not related to the distance between the Sun and Earth.
Option-4: Angstrom is also not the correct answer. It is a unit of length equal to 10^-10 meters, commonly used to measure wavelengths of light. It is not used to describe the distance between the Sun and Earth.
The mean distance from the Sun to the Earth is called an Astronomical Unit (AU). It is defined as the average distance between the Earth and the Sun, which is approximately 149.6 million kilometers. The AU is commonly used in astronomy to measure distances within our Solar System.