Concept:A light-year is a unit of distance used in astronomy to measure vast distances between celestial objects.Explanation:A light-year is the distance light travels in one Earth year.The speed of light is constant at 3×108 m/s.In one year, light travels about 9.7 trillion kilometres or 6 trillion miles.It is not a measure of time, speed, or light intensity.Astronomers also use the astronomical unit (AU) for distances within a solar system and the parsec (pc) for even larger scales (1 parsec ≈ 3.3 light-years).Answer:C. distance