The only thing absolute in this universe according to Einstein is the speed of light, rest everything is relative. Light travels at the speed of about 300,000 km per second and it is always same throughout the universe. A light- year is the distance light travels in one Earth year. On the scale of the universe, measuring distances in miles or kilometers doesn't cut it therefore, astronomers measure the distances of stars and other astronomical distances in the time it takes for light to travel to us. Hence option (d) is the correct option.