I don't like the wording of any of the choices on the list.
SONAR generates a short pulse of sound, like a 'peep' or a 'ping', focused in one direction. If there's a solid object in that direction, then some of the sound that hits it gets reflected back, toward the source. The source listens to hear if any of the sound that it sent out returns to it. If it hears its own 'ping' come back, it measures the time it took for the sound to go out and come back. That tells the SONAR equipment that there IS a solid object in that direction, and also HOW FAR away it is.
RADAR works exactly the same way, except RADAR uses radio waves.
