Answer:
Radio wave
Step-by-step explanation:
If a wave has a wavelength of 1 kilometer, it must be a radio wave. The wavelength of radio waves lies in more than 1 kilometers. These waves have least frequency. Radio waves have less energy because energy and wavelength have inverse relationship.
Radio waves are mostly used in radar, mobile radio communication and nevigation system.
Hence, the correct option is (3) " radio wave ".