The true statement about a light year is:
It is used to measure long distances in space.
A light year is the distance that light travels in one year, which is approximately 5.88 trillion miles (about 9.46 trillion kilometers). It is commonly used in astronomy to express distances between stars and galaxies. The other statements are incorrect: a light year is longer than an astronomical unit (the average distance from the Earth to the Sun), and it does not measure time periods.