Kilometer --> mile --> AU --> light-year
Explanation:
A Kilometer is equal to 1,000 meters and a mile is equal to 1.609 kilometers. Both kilometer and mile are unit to measure distance. A Light year is the distance that light travels in 1 year while AU (also know as Astronomical Unit) is the distance between the Sun and the Earth. An AU is 149,600,000 km. A light year is approximately 9,500,000,000,000 km. So, AU is much smaller than Light year. Hence, answer is C , Light year being the largest unit and kilometer being the smallest.