Official Explanation
In this question you are asked to compare two measurements, one given in kilometers and the other in meters. It would be easier to compare these measurements if they were both given in meters or both given in kilometers.
If you choose to convert the distance that Runner B ran from meters to kilometers, you need to use the conversion 1 meter is equal to 1/1000 kilometer. Since B ran 800 meters, it follows that B ran 800(1/1000), or 4/5 kilometer, which is the same distance that A ran.
If you choose to convert the distance that Runner A ran from kilometers to meters, you need to use the conversion 1 kilometer is equal to 1,000 meters. Since A ran 4/5 kilometer, it follows that A ran 4/5(1000), or 800 meters, which is the same distance that B ran. Either way, A and B ran the same distance, and the correct answer is Choice C.