Final answer:
A light-year measures the distance light travels in one year, equal to about 9.5 trillion kilometers. It is a unit of distance, not time, despite including 'year' in the name and is commonly used in astronomy to describe vast cosmic distances.
Step-by-step explanation:
The correct answer to the question is option (b): It measures the distance light travels in a year. A light-year is a unit used by astronomers to measure cosmic distances, and it represents the distance that light, traveling at a speed of approximately 300,000 kilometers per second, covers in one Earth year. To put it into perspective, a light-year equals roughly 9.5 trillion kilometers.
The light-year is a vital measurement in astronomy because the universe is so vast that close objects are still incredibly far away, with the nearest star to our solar system, Proxima Centauri, being over 4 light-years away. Considering that light speed is the fastest speed possible in the universe, using light-years makes it more manageable to express and understand these immense distances.