The correct answer is B. 0.5 km. That is because if you convert them all to meters you will see that 0.5 km is 500 meters. 50 meters is smaller than that, while 50000 millimeters is actually 50 meters in size. 5000 centimeters is also 50 meters when converted. Half a kilometer is 500 meters which is 10 times larger than all others.
