Answer :
The largest practical unit for distance is the light year.
1. A light year is the distance that light travels in one year in the vacuum of space, which is approximately 9.46 trillion kilometers. This unit is commonly used in astronomy to measure vast distances between celestial objects.
2. The parallactic second is a unit used in astronomy to measure the parallax angle of a celestial object. It is not a practical unit for distance measurement on a large scale like the light year.
3. An astronomical unit is the average distance between the Earth and the Sun, which is approximately 149.6 million kilometers. While it is a useful unit within our solar system, it is not as expansive as the light year.
In conclusion, the light year is the largest practical unit for measuring distance in astronomy due to its vast scale and convenience for expressing distances across the universe.
