Right answer: It is used to measure long distances in space
A light year (
) is a distance unit that is approximately
.
By definition it is the distance that a photon would travel in vacuum during a Julian year (365.25 days) at the speed of light (299 792 458 m/s) at an infinite distance from any gravitational field or field magnetic.
In other words:
It is the distance that light can travel in a year.
This unit is used to measure large distances in space. Therefore the correct answer is D.