Explanation

In deep-space astronomy, distances between interstellar systems and galaxies are too vast to be practically conveyed using conventional terrestrial units like kilometres or miles.

– A light-year is defined as the distance an electromagnetic wave travels through empty space in one Julian year (approx. 9.46 trillion kilometres).
– Under Einstein’s Special Theory of Relativity, the speed of light in a vacuum (c) is a universal physical constant, precisely measured at 299,792,458 metres per second, unaffected by the motion of the source or observer.
– Because this velocity never varies across space and time, it provides an absolute, unchanging standard metric for cosmic distances.

Therefore, astronomical distances use light-years because the speed of light is always same — Option D.