Which one of the following measurement represents the longest distance 5m, 10cm, 50mm, 1km

First you need to convert them to one common measurement ... I'll use cm for the measurement.

1m = 100 cm 10 cm = 10 cm 50 mm = 0.1 cm 1 km = 100000 cm

Using this info to convert gives

500 cm 10 cm 5 cm 100000 cm

The smallest is 50 mm which is equal to 5 cm